Optimizing MySQL On A Busy Server (4 Gigs Of Ram)

it's getting to the point where I need to optimize MySQL to better handle a busy server.

These days it seems MySQL is using 30% - 60% CPU almost constantly. But, the good news is that I've got 4 gigs of ram on this box and their seems to be an access of 600mb free constantly throughout the day.

Is it possible to tweak MySQL to a little more RAM dependent and take some of the load off the CPU? It seems MySQL wont go over 300mb of ram at any given time.. I'm already working on optimizing the SQL Querys on the web page itself.

my.cnf (4.1.22-standard):

# Default to using old password format for compatibility with mysql 3.x
# clients (those using the mysqlclient10 compatibility package).
# old_passwords=1

# [mysql.server]
# user=mysql
# basedir=/var/lib

# [mysqld_safe]
# err-log=/var/log/mysqld.log
# pid-file=/var/run/mysqld/mysqld.pid

[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
safe-show-database
old_passwords
back_log = 75
max_connections = 600
table_cache = 128
thread_cache = 32
wait_timeout = 60
interactive_timeout = 80
connect_timeout = 60
tmp_table_size = 64M
max_heap_table_size = 64M
max_allowed_packet = 64M
max_connect_errors = 10
read_rnd_buffer_size = 524288
bulk_insert_buffer_size = 64M
query_cache_limit = 8M
query_cache_size = 100M
query_cache_type = 1
default-storage-engine = MyISAM
local-infile=0
thread_concurrency = 4

[mysql.server]
user=mysql
basedir=/var/lib

[mysqld_safe]
err-log=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid
open_files_limit = 8192

[mysqldump]
quick
set-variable = max_allowed_packet=16M

[myisamchk]
set-variable = key_buffer=256M
set-variable = sort_buffer=64M
set-variable = read_buffer=16M