So primarily for my new computer I want to use Final Cut Studio for editing with HD videos. I know that I want a Quad-core processer, but I'm not sure whether I should go for an iMac or a Mac Pro.

Budget is a concern, as I want the most powerful system for the best price. I am also looking on the secondary market for used systems.

What would get me the most bang for my buck? I know I atleast want a quad-core with 8GB of ram. I wouldn't be opposed to buying the ram myself. Is there a system that's going to give me the most bang for my buck?