Wouldn't go lower than GTS250, GTX 260 is a nice card. Personally I'd go for GTX275, GTX285 just above that if you have a bit more to spend. GTX295 is a dual-GPU card; big powerdraw and heat, but big performance.
Radeon HD5750 and 5770 are the new mainstream from ATI, 5850 and 5870 for the high end (5850 would be my choice, 5870 if more cash). 5970 is their dual-GPU offering, same drawbacks as for GTX295.
Any of those cards should give you what you need to enable some anti-aliasing, which is what gets rid of the 'jaggies'.