If you're gonna play at low details with 1024x768 or something then even a 9800GT will do
. Anyway, a GTS250 should be good if you're going for 1680x1050 or lower with medium details. I say though to go with a Radeon 5770 as it's faster and is a DX11 GPU
. If that's too much for you then go with the 5750 instead.