Well, we need more info here, tell us your FULL specs first, including your monitor and budget...
Running win7-64 SP1 on an i7 870 Quad core 2.93 GHz, 8 GB DDR3/1333 ram, 1 TB SATA HDD, 24 IN WS Dell 2407WFP HDMI usually at 1920x1200 with a second monitor Dell 1800FP @ 1280x1024. Current graphics card is a GT240 512 MB DDR5
Any new card will get a new power supply to feed it. Mostl likely a 650 Watt. Thanks.