Firstly, Every current card (on their own) will struggle with oblivion. Hell, some of the midrange cards would break into sweat with Morrowind turned up to its full settings (with the better mods of corse). Its just the nature of the ES series.
The 6600GT would surfice for practily everyting atm, as would the X1600. Wich is actualy a tad better than the 6600GT, closer to a 6800. I know, i had one. Llwyd now uses it, and is going crossfire very soon. I would recomend you get a X1600XT from a reputable retailer.
Edit: And couple it with the omega drivers, not the cats.