I don't mean RAID, although it can help.
I mean specific disks (or RAID-1 pairs) mounted at specific mountpoints. You'll want to split off, in order of benefit
* main DB data storage
* DB's transaction log (relevant to INNODB, not ISAM/myISAM)
I can only recommend you read the usual DBA literatureabout increasing the numbers of spindles. It's a classic, and if you do it right, the thoughput will jump.