You want SCSI3.. its 160MB a sec max.
ATLASII is a 10,000 RPM HD (fastest IDE is 7,200). A 18 gigger is like 200.
Cheetah is a 15,000 RPM HD. 18 gigger is like 300.
Both needs a SCSI card of Scsi2 or better, Tekram makes some nice ones.
I dont have SCSI tho so if anything is wrong, im sorry 😉