I got a EVGA GTX550Ti for around $140 at microcenter, and it plays skyrim with everything on the highest settings with no FPS problems, only have 4gigs of ram too. The 550ti is a GDDR5 1 gig card.
I was using a 9800GTX+ before my 550ti, and the 550 just blows it out of the water. Its insane how much better the card is.
Also i was reading about these cards before i got mine, a 550 will preform better than a 640 and so on. I originally picked up a 640, came home powered it on, did some FPS testing and reading on the internet and found out the 550ti was WAY better than my 640, so i took it back and got the 550 and have been happy since. Id either get a 660 or a 550. Dont bother with the 640's or 650's, 540's or any of the 400s.
Im assuming you have a 650-700+ watt Power Supply?