The last month we started having delays processing orders in the backend in a production e-Commerce Website.
We adjusted the system to this config and we the website runs at a really good performance now.

We calculated this values using this formula:
key_buffer_size + (read_buffer_size + sort_buffer_size) * max_connections = Total Memory

Website Details
Magento E-Commerce Website
Running in a:
Dell Server
CPU cores: Two
Processor(s): 1x Dual Xeon
Clock Speed: >= 2.33GHz
RAM: 2 GBytes
Disks: 1×250 GB SATA

Managing:
13.651 Orders in the last 6 months.
24.403 Customers
1.196 Products

Download mysqltuner.pl
More Info:
The MySQL Performance Blog http://www.mysqlperformanceblog.com/2006/05/17/mysql-server-memory-usage/
Stack Overflow: http://stackoverflow.com/questions/1178736/mysql-maximum-memory-usage

Reports
#
#
# Default Configuration
#
#

/etc/my.cnf
innodb_buffer_pool_size = 128M

mysql> SHOW VARIABLES

+-----------------------------------------+------------------------+
| Variable_name                           | Value                  |
+-----------------------------------------+------------------------+
| innodb_buffer_pool_size                 | 134217728              |
| key_buffer_size                         | 8384512                |
| max_allowed_packet                      | 1048576                |
| sort_buffer_size                        | 2097144                |
| read_buffer_size                        | 131072                 |
| read_rnd_buffer_size                    | 262144                 |
| tmp_table_size                          | 16777216               |
| myisam_sort_buffer_size                 | 8388608                |
| query_cache_size                        | 0                      |
| query_cache_type                        | ON                     |
| query_cache_limit                       | 1048576                |
| thread_cache_size                       | 0                      |
| max_connections                         | 500                    |
| innodb_lock_wait_timeout                | 50                     |
| wait_timeout                            | 28800                  |
+-----------------------------------------+------------------------+

./mysqltuner.pl
——– General Statistics ————————————————–
[--] Skipped version check for MySQLTuner script
[OK] Currently running supported MySQL version 5.1.37
[OK] Operating on 32-bit architecture with less than 2GB RAM

——– Storage Engine Statistics ——————————————-
[--] Status: -Archive -BDB -Federated +InnoDB -ISAM -NDBCluster
[--] Data in MyISAM tables: 923M (Tables: 190)
[--] Data in InnoDB tables: 1G (Tables: 549)
[!!] Total fragmented tables: 573

——– Performance Metrics ————————————————-
[--] Up for: 15s (147 q [9.800 qps], 9 conn, TX: 289K, RX: 75K)
[--] Reads / Writes: 100% / 0%
[--] Total buffers: 154.0M global + 2.7M per thread (500 max threads)
[OK] Maximum possible memory usage: 1.5G (73% of installed RAM)
[OK] Slow queries: 0% (0/147)
[OK] Highest usage of available connections: 0% (2/500)
[OK] Key buffer size / total MyISAM indexes: 8.0M/193.3M
[OK] Key buffer hit rate: 100.0% (52 cached / 0 reads)
[!!] Query cache is disabled
[OK] Sorts requiring temporary tables: 0% (0 temp sorts / 71 sorts)
[OK] Temporary tables created on disk: 1% (1 on disk / 70 total)
[!!] Thread cache is disabled
[OK] Table cache hit rate: 88% (55 open / 62 opened)
[OK] Open file limit used: 0% (16/2K)
[OK] Table locks acquired immediately: 100% (366 immediate / 366 locks)
[!!] Connections aborted: 11%
[!!] InnoDB data size / buffer pool: 1.4G/128.0M

——– Recommendations —————————————————–
General recommendations:
Run OPTIMIZE TABLE to defragment tables for better performance
MySQL started within last 24 hours – recommendations may be inaccurate
Enable the slow query log to troubleshoot bad queries
Set thread_cache_size to 4 as a starting value
Your applications are not closing MySQL connections properly
Variables to adjust:
query_cache_size (>= 8M)
thread_cache_size (start at 4)
innodb_buffer_pool_size (>= 1G)

#
#
# New Configuration
#
#

/etc/my.cnf
innodb_buffer_pool_size = 384M
key_buffer = 256M
query_cache_size = 1M
query_cache_limit = 128M
thread_cache_size = 8
max_connections = 400
innodb_lock_wait_timeout = 100

mysql> SHOW VARIABLES

+-----------------------------------------+------------------------+
| Variable_name                           | Value                  |
+-----------------------------------------+------------------------+
| innodb_buffer_pool_size                 | 402653184              |
| key_buffer_size                         | 268435456 			   |
| max_allowed_packet                      | 1048576                |
| sort_buffer_size                        | 2097144                |
| read_buffer_size                        | 131072                 |
| read_rnd_buffer_size                    | 262144                 |
| tmp_table_size                          | 16777216               |
| myisam_sort_buffer_size                 | 8388608                |
| query_cache_size                        | 1048576                |
| query_cache_type                        | ON                     |
| query_cache_limit                       | 134217728              |
| thread_cache_size                       | 8                      |
| max_connections                         | 400                    |
| innodb_lock_wait_timeout                | 100                    |
| wait_timeout                            | 28800                  |
+-----------------------------------------+------------------------+

./mysqltuner.pl
——– General Statistics ————————————————–
[--] Skipped version check for MySQLTuner script
[OK] Currently running supported MySQL version 5.1.37
[OK] Operating on 32-bit architecture with less than 2GB RAM

——– Storage Engine Statistics ——————————————-
[--] Status: -Archive -BDB -Federated +InnoDB -ISAM -NDBCluster
[--] Data in MyISAM tables: 926M (Tables: 190)
[--] Data in InnoDB tables: 1G (Tables: 549)
[!!] Total fragmented tables: 572

——– Performance Metrics ————————————————-
[--] Up for: 13s (141 q [10.846 qps], 11 conn, TX: 396K, RX: 59K)
[--] Reads / Writes: 97% / 3%
[--] Total buffers: 659.0M global + 2.7M per thread (400 max threads)
[OK] Maximum possible memory usage: 1.7G (85% of installed RAM)
[OK] Slow queries: 0% (0/141)
[OK] Highest usage of available connections: 1% (4/400)
[OK] Key buffer size / total MyISAM indexes: 256.0M/193.7M
[OK] Key buffer hit rate: 100.0% (52 cached / 0 reads)
[OK] Query cache efficiency: 23.6% (29 cached / 123 selects)
[OK] Query cache prunes per day: 0
[OK] Sorts requiring temporary tables: 0% (0 temp sorts / 53 sorts)
[OK] Temporary tables created on disk: 1% (1 on disk / 54 total)
[OK] Thread cache hit rate: 63% (4 created / 11 connections)
[OK] Table cache hit rate: 86% (46 open / 53 opened)
[OK] Open file limit used: 0% (16/2K)
[OK] Table locks acquired immediately: 100% (265 immediate / 265 locks)
[!!] Connections aborted: 9%
[!!] InnoDB data size / buffer pool: 1.4G/384.0M

/* Shell Commands*/
mysqladmin -u root -p shutdown
mysqladmin -u root -p'oldpassword' password newpass
/* Users */
// Any host
CREATE USER 'user1'@'%' IDENTIFIED BY 'user1';
GRANT ALL PRIVILEGES ON MyDatabase.* TO user1@'%' IDENTIFIED BY 'user1' WITH GRANT OPTION;
SET PASSWORD FOR 'user1'@'%' = PASSWORD('123456');

DROP USER user@'%';
//Specific host
CREATE USER 'user1'@'192.168.1.155' IDENTIFIED BY 'user1';
GRANT ALL PRIVILEGES ON MyDatabase.* TO user1@'192.168.1.155' IDENTIFIED BY 'user1' WITH GRANT OPTION;
SET PASSWORD FOR 'user1'@'192.168.1.155' = PASSWORD('123456');

DROP USER user@'192.168.1.155';

/* Text Field, instead of varchar
for unlimited text field use TEXT var instead of VARCHAR(number)
Not recommended for large databases
*/

/* dump database in /var/www/File-2009-10-28_20:34.sql */
mysqldump -u'user' -p'password' name_database > /var/www/File-$(date +%F_%R).sql

/* Rename a database */
RENAME DATABASE Db_name TO new_db_name;

/* Export Data */
//Export Rows Into A SQL File
mysqldump --user root -p --databases mydatabase --tables 'mytable' --where 'id != 10' > fileUploads3.sql

//Export Rows Into A CSV File
SELECT *
 INTO OUTFILE '/Users/nacho/file1.csv'
 FROM mytable;
SELECT *
 INTO OUTFILE '/Users/nacho/file2.csv' FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"'
 FROM mytable
 [...];

/* Import Data */
/*
admin1Codes.txt
ES.00	Ceuta
ES.07	Balearic Islands
ES.27	La Rioja
[...]
Relative Path
*/
LOAD DATA LOCAL INFILE './admin1Codes.txt' INTO TABLE admin1Codes (code, name);
/* Absolute Path */
LOAD DATA INFILE '/Users/nacho/Documents/admin1Codes.txt' INTO TABLE admin1Codes (code, name); 

/* Run Commands From Shell */
mysql -u'root' -p'root' -e'SHOW DATABASES'

mysql -u'root' -p'root' database < script.sql