Wouldn't buss speed be far more important than access speed for this?
I'm thinking about putting one of the old 40 or 60 gb in for purely scratch disking.
PS CS indicates that some 16 bit .psd images are near a gig each so with 30-50 history states that should be enough
Sorry about jumping into an old post but I just found it and this is an issue that I've spent at least a bit of time think over.