I haven't looked into a GPU since buying my last one several years ago so I don't know if it's still the case, but Nvidia used to have better drivers than ATI which would sometimes have comparability issues and performance problems. Last I heard I think they were getting better.

That aside, check out Tom's Hardwares list of the best GPU's for the price. They always include a list which lists comparable ATI and Nvidia cards so you can compare them based on price and performance pretty easily. Use it when you compare prices to get the best card you can afford. You might be able to find some sales or deals so you should shop around.