For samples and user data I chose the rather inexpensive WD Caviar Black (7200rpm, SATA 3, 64MB of cache).
My PC is very new and I still did not install any sampler software. Actually I am waiting for a Firewire PCI express card that will allow to connect my old Firewire audio interface, but the shops and the post are slow here.
I am pretty sure that I will be able to run my sample banks without any problem (I am not a hardcore gigabank user and had a 5400rpm laptop before).
You may have a look at SAS drives (15000rpm) though they are quite expensive, moreover they either require special motherboards or PCI express SAS controllers.
Check also the WD VelociRaptor (10000 rpm, SATA 3) which is slightly less expensive.
If you choose such a fast drive, check for potential heat / silence problems. Also, I have no idea if they are useful or even adapted to audio work.
Finally, you can also have a look at RAID 0 configurations too (same remark: not sure if it is required for streaming samples).