I have three extra Western Digital 250gb SATA drives that I'd like to put to
use.

My thought is to put them into a "rack" and make a 750gb RAID array out of
them for additional storage on my PC, probably via a USB connection.

I've heard of Drobo but is there a less expensive rack I can buy that will
allow me to slide these three (and possibly a 4th) drives into, then RAID
them into a 750gb virtual drive, and occasionally connect to my desktop for
data archiving?

It goes without saying that the rack system must be reliable. I'd appreciate
any leads.

TIA---