However, you should check different possible bottlenecks :
* cpu usage, network interface
Just to be sure, try auditing the cpu(s) usage. Also, how far your i/o are from your network limits ?
With many little files, check out what filesystem would possibly do the best job. The new reiser file system is supposed to be quite good for this, but is still a little bit experimental.
* raid subsystem
If you cannot go for SCSI, at least try devices with TQ abilities (both for your controller and your drives).
As it has already be said, raid 5 is not quite adequat if write performances are an issue.
If possible, you should try raid 10 or 50 just to know if it helps or not (actually, for exhaustive test purposes, you could as well test raid 0, just to know precisely what is the precise impact of your raid configuration on your performance issue).