I'd go with an Nvidia GEForce 6200. It's pretty basic, and won't do a lot of the really shiny stuff, but it's more than good enough to run Windows Vista Aero, and it'll keep up with most games out nowadays (but don't expect to run Oblivion at full graphics)
It'll run you between $60 and $80.
Now, if you want bare-bones, absolute minimum, go for an Nvidia GEForce FX 5200. It's got half the memory (128 MB), but it should get you by, and it'll only run you $45 or so.

If it were me, even if I was on an absolute bare-bones budget, I'd go with the 6200. It's an extra twenty bucks, which isn't too much, and the difference will be VERY noticeable.