I use 15K RPM hard drives for my CS2 scratch disk. In fact, I use 3 15K RPM drives in a RAID 3 setup for the scratch alone. There is a major speed boost with the faster hard drives over 7200 RPM, but I have never done a RAID with 7200 RPM drives so I would not know.
How noisy are they? I don't like slow computers, but I don't like noisy ones either, and prefer not quite leading edge performance if it quietens things down a lot. Of course I don't have volumes of shots to process in as short a time as possible, either.
I want to go back to a fanless, diskless desktop with remote storage.