CPU + Mobo + RAM + Liquid Cooling: <$300
The GPU and PSU I'm planning on getting as an upgrade to my current: About $800
Kinda a shame I don't live in the US, could get my CPU and such over there on sale as my dad was working there at the time, now I'm stuck with Aussie prices on 2 560Tis and a 1000W PSU. Sure I don't NEED it, but I WANT it, and I do need an upgrade to play BF3 on Max settings.
This is of course going along the lines that you already have a PC that you can salvage things from. If you don't, it will cost a fair bit more to get a screen, case, PSU, Mouse&Keyboard, Sound Card ect. that you could easily just use an old PCs of instead.