what does he mean by changing to the interface? That could mean that he's getting an ATA to SATA converter which really doesn't increase bandwidth. It looks like you should be doing SATA though. And for that matter, you should probably have two of the hard drives in SATA onboard RAID for increased performance. if you're not using RAID though, the SATA performance won't be much better than the ATA performance. But hey, again this is a workstation, so you should have RAID.
And Marc: I think he Already is using PATA; basically all modern computers have two IDE channels and he's maxed out at four drives meaning two drives per channel. again, that's another performance hit that he needs to counter (file transfer between hard drives can be very, very slow)
in short: GET RAID! I have it and it's cheap and fast
Pentium 4 2.4C
1GB Kingston HyperX PC4000 Dual-Channel
2x80GB Western Digital 7200RPM 8MB Cache RAID 0
ATI All-In-Wonder 9600 Pro
NEC 19\" CRT