One suggestion from another list (I've shared your pain) is to test an alternate filesystem. I'd think this would have minimal impact for a database, as you're largely pumping data in and out of a small number of files, but it's possible. See [link|http://www.softorchestra.com/mailman/listinfo/linux-consultants|http://www.softorchestra.com/mailman/listinfo/linux-consultants] for details.
\r\n\r\nReiserfs does better than ext2fs in some circumstances. The SGI 64-way system featured in LJ a month or two back used SGI's journaling FS, don't remember if that's XFS or JFS. Performance over Reiser and ext3 was stunning. Might give that a shot.
\r\n\r\nAlso: RAID5 probably slows you. RAID 1-0 is a better performance bet, and offers redundancy.