The GT 240 is overpriced and underpowered.
The GTS 250 competes with the HD 4850/4770 in performance and all are equally as good of a deal in the $100-120 price range; anything more than that and it's better to go for an NVidia GTX 260 216sp or ATI HD 4870/4890. If you are interested in DirectX 11 try checking out the new five series from ATI as well.
A GTS 250 is probably your best bet:Galaxy 25SFF6HX1RUI GeForce GTS 250 512MB 256-bit DDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card - Retail
Don't pay over $120 for a GTS 250.