As already noted, the storage engine selected is the main factor affecting MySQL's performance. Should you use MyISAM or InnoDB, that depends on your workload.
If in your case SELECTs are fast to do and there's not much write activity to DB, MyISAM is not bad at all. But if you have even moderate write activity and some long-running SELECTs, please be aware that MyISAM only supports table-level locks. During a slow SELECT all write activity will be queued and that can pretty soon lead to all kind of nasty problems.
InnoDB supports row-level locks, transactions and multiversioning, so even a heavily mixed read/write operations will not slow it down nearly as much as MyISAM.
Also make sure that whatever the storage engine is in use, it is properly tuned. For MyISAM *key_buffer_size* and *table_cache* are the most important values to tune, for InnoDB the first thing to adjust is *innodb_buffer_pool_size*.
But both of the storage engines have their own gotchas: for example, only MyISAM supports full-text indexing and InnoDB table space does not shrink, so in an actively updated table where there's lots of deletes/updates/inserts going on, you need to dump to table contents and suck them back in every now and then.