You should state your target resolution as well. going from 1680x1050 to 1920x1200/1080 can make a difference.

If you want, AnandTech keeps a profile of GPUs with their performance benchmarks for comparision. I read AnandTech often and the content they create is interesting to read, to say he least. Skyrim isn't on their benchmark list, yet, but other games should give you a frame of reference for what to expect.

I've been living on my Radeon 5770 for the past 2 years fairly. Then again, I don't play many hardcore games like FPSes. Still, you can probably get by on a 6850-class or so video card if that's the case. Perhaps even a 6770-class card. (I'm not sure what the Geforce equivalents are off the top of my head. I've been using Radeons in my desktops since 2001. Only my laptops have used something else, those being a Geforce FX in 2003, Intel GMA950 in 2007, and Intel GMA4500 in 2010.)