Optimizing MySQL for Read-Only?
MyISAM ROW FORMAT
If the table is a lot of VARCHAR fields, I would love to suggest the following
For every MyISAM table you load, do this beforehand
ALTER TABLE mytable ROW_FORMAT=Fixed;

This will increase query performance up to 15-30% without changing anything else. The side effect of doing this is having bigger tables and bigger indexes.
I have earlier posts on this:

May 10, 2011 : What is the performance impact of using CHAR vs VARCHAR on a fixed-size field? (TRADEOFF #2)
Aug 12, 2011 : Which DBMS is good for super-fast reads and a simple data structure? (Par 3)
Sep 20, 2011 : Best of MyISAM and InnoDB
Jan 03, 2012 : Optimized my.cnf for high-end and busy server (Under the heading Replication)

INCREASED RAM
If you have many indexes, you may want to increase RAM on the DB Server and then attempt to preload all MyISAM index pages into the key buffer.
First compute how big a key buffer you would like (in GigaBytes)
SELECT CEILIING(SUM(index_length)/POWER(1024,3)) SuggestedKeyBufferSize
FROM information_schema.tables
WHERE engine='MyISAM' AND table_schema NOT
IN ('information_schema','performance_schema','mysql');

If SuggestedKeyBufferSize can be set up for the DB Server, then do the following (example 4G)
STEP 01) Set this in /etc/my.cnf
[mysqld]
key_buffer_size=4G

STEP 02) preload all MyISAM indexes
See my post on how to do this : MySQL warm procedure