Purchasing drives for a RAID is really no different than purchasing them for regular use. You'll basically be looking for speed, reliability and possibly noise and heat if those are issues.
RAID requires at least two hard drives to be considered a RAID, you need two hard drives that are identical, meaning the same brand, the same size, the same interface, the same type. You can't make a RAID with one drive.
Depends which RAID array you wish to setup, it can get a bit tricky though. Your motherboard should probably have built in RAID 0/1/0+1 controllers, and then you install the appropriate drivers, and configure it properly.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w