I will suggest you to buy an ATI RADEON GPU as it is better than NVIDIA in price and performance of ATI GPU'S are almost near to NVIDIA'S.ATI'S new series is much much better than the NVIDIA's.If you want better GPU in low cost then you must go with ATI else your games will sucks if u purchase any NVIDIA's gpu in this range......
So its totally yours choice.Best off luck...
The gpu's of ATI with in your range will b -----
ATI 5670HD DX 11
ATI 4770
ATI HD4870
Radeon HD 5770
ATI HD 5750