table_cache is the most useful configuration directive to change. Each time MySQL accesses a table, it loads the table into cache. If you've got a high number of tables, it's faster to have them cached.
Take a look at your server variables by running:
and have a look for the variable
open_tables. If this is the same as your
table_cache value, and
opened_tables keeps going up, then you need to increase the
table_cache value in your configuration file. You'll find a balance by experimenting with these variables during peak times. You want to configure it so that at peak times, there are a low amount of
opened_tables even after the server has been up for a long time.
key_buffer_size is also a good variable to experiment with. This variable affects the index buffer size, and making this variable bigger increases MySQL's index handling speed. You can look at the variables with the
show variables; command again, and compare
key_reads. Ideally, you want the ratio between these two variables to be as low as possible, and you can do this by increasing the size of the
key_buffer_size. If you set this variable higher, you will have less writes and reads directly to and from the disk, which was your main concern.