I have played on above medium settings on a core2duo. Quad core should be fine with any gpu in it, as it's mostly gpu based. If your cpu gets taxed set physics to the video card instead of cpu. I started this game on an 8800GTS 320mb, then GTX280, ATI 6870, GTX580, now GTX580 x2. I have 2 other machines with core2duo's and gtx285's that play on medium settings. Don't cheap out on a psu, but you don't need to go with 1kw psu either. I have a GOOD psu(brand wise), pc power & cooling, only a "mere" 750w, that powers my entire system that has dual gtx580's, 2 ssd's, one regular hdd, cd-rom. Those PSU's run about $80 from their website.

Here's my breakdown of GPU quality for nVidia:

x40 Series - budget
x60 Series - budget
x70 Series - budget
x80 Series - Great
x85 Series - better
x90 Series - Overpriced

I would not even consider a gt440 video card, or gtx470, it's x80 or nothing for me. But each person has their preference.

BTW the game looks super sweet on ultra high settings at 5760x1200 resolution...
http://www.youtube.com/watch?v=zByk4YbR51M