I would say the GTX 650 TI would be one of the best cards for that price range. If you do find a GTX 560 TI on sale that would be another option and for the ATI side then you can go for the HD 7770 or the HD 7850. Anyways it would be best if you list your entire build so we can determine how much wattage you need.
The 7950 is a good value in my opinion for 300. If you don't have quite that much you can always get a 7870. It'a about 80 bucks less and is almost as good as a 7950. As for Nvidia I'm sure there are some good budget cars but I haven't looked into them.