When I was printing with onyx postershop, in-house about 5 years ago, I upgraded to a Dell Dual Xeon processor with 2 gigs of ram and dual hard drives. Since it was a RIP only computer, connected to a network and server, it had no problems handling large 500+mb files and running 2 printers.
Today, I would take a serious look at the Signburst PC's because Casey seems to be genuinely interested in making good systems for the graphics industry.
I would stay away from anything store bought and make sure you get some sort of warranty/service agreement too. Computers never seem to break down when you don't need them
Checkers