Before you buy a video card make sure you have the Power Supply to run it. I was shocked yesterday when I discovered that the GTX260 requires a whopping 630Watt power supply where as the HD4870 only requires a 450 Watt power supply. The DDR5 ram on the HD4870 along with architecture of its R700 chip really makes it power efficient. It runs around the same speed as the GTX260 however it does so with 180 Less Watts of power.
Look under 'Specifications' on these links for the power supply ratings:
If you do get a GTX260 besure to get the one with the 216 Stream Processors because that one is faster than the 4870 in a good deal of benchmarks. Especially in games like Call of Duty World at War. EVGA and XFX are probably going to be the best companies to buy from.