I hope you haven't bought a motherboard yet, or if you have, that you had the sense to buy one with PCI Express x16 slots.  If that's the case, then you have two options: nVidia or ATi.  Personally, I prefer nVidia.  They actually know how to write drivers, and their cards typically outperform the equivalent ATi card.  I would suggest looking around the internet for benchmark tests (I'm too lazy to look myself), and I recommend starting at http://www.tomshardware.com .  I personally have a PCI-E x16 LeadTek GeForce 7800GTX.  When I bought it, it was $500, now it's much, much cheaper.  My brother's 7900 can run F.E.A.R. at the standard LCD monitor resolution of 1280x1024 with high 60-70fps framerates (and I do believe he maxed out all of the settings).

For a LeadTek board, check out:
http://www.newegg.com/Product/Product.asp?Item=N82E16814122012

And for pretty much the same card with ASUS (another great company):
http://www.newegg.com/Product/Product.asp?Item=N82E16814121025

If you don't mind paying for the best of the best:
http://www.newegg.com/Product/Product.asp?Item=N82E16814121033

I also recommend perusing http://www.pricewatch.com .  There are more internet PC component sellers than newegg, I just forget a lot.  =)

The two links I gave to the two 7950s are to GT cards, not GTX.  GTX cards appear like the third link, and have elaborate and effective cooling systems.  They also clock higher and are a bit faster (and are thusly more pricey) than their GT brothers.  I would recommend searching for GTX versions instead of GT, though GT cards aren't necessarily bad.  NewEgg seems to have a merchandise shortage, so you'll probably have to search http://www.pricewatch.com .

If you're interested in ATi, you're on your own, 'cause I don't care for ATi.  =P