Two things to consider here. First is your power supply. There should be a little sticker on the side of your power supply that will list total wattage, as well as amp values for individual volts. Get us the amp info for all +12V. Im guessing if it handled the 8800GTS, it should be OK for the GTX260, but its always good to check.
Second thing is your processor. While it kicked ass in 2005, today its well behind the curve. To be honest, your probably not going to see alot of improvement moving from your 8800GTS. I would just hold off and save up for a whole new build.