I was running Skyrim OK with a 9800GT, then upgraded to a GTX 560 (2GB, not a ti). I was able to run it pretty well before on mostly medium/high settings, now I'm on mostly high and ultra with a uGridsToLoad of 7, and the lite version of Skyrim HD. This is with nVidia 3D vision though, which just about doubles the strain on your graphics card, so I'd imagine most people could get away with something a bit weaker. I could be wrong, but this game doesn't seem much more taxing than New Vegas.
This is the card I'm running. The EGVA nVidia GTX 560 2GB....I only have an AMD dual core running @2.49GHz and only 2Gb ram...
I'm able to run on high settings with at least 30 fps and higher everywhere & up to 60 fps in most wilderness areas. It drops by about 10 fps below that in Ultra.
Also, running Game Booster 3 program so that helps by temporarlily shutting down background system resources.
I'm happy with my GPU.
Some folks like ATI Radeon Cards, some like nVidia.
I upgraded from a nVidia 9600GT 512Mb card. I could run Skyrim in low settings, but it would choke after a while.