putting the total upgrade cost at $650.
That's right about what I was coming out with while speccing it out, without a new heatsink. If I splurge on the motherboard for Phenom, I can get one for $400 - possibly as low as $335 if I get a board that only supports x16/x4 on its PCIe slots. Plus, I just heard about the next architecture, and it sounds like it's coming out on the AM3 socket, while Intel appears to release a brand new socket every revision (socket 1155 for Sandy Bridge?).
Thanks for your help, I should have looked farther into everything before posting this thread. Gonna go with the Phenom so that I can possibly keep future compatibility.