I am happy as can be with my GTX460. The GTX560 is only 1 step up. I would keep the 460 myself especially since a return usually means a restock fee.
Edit: That 460 is a great overclocker and will equal or surpass a stock 560 easily.
Are you sure about this? I read lots of reviews and charts and it seems the 560 is ahead of the 460. The stores here in Japan do not charge a restock fee. As long as you have the reciept you can return for full refund.
If a 460 can truely be overclocked and bypass a 560 ti than I will keep the 460.
A quality 500w unit should be fine and leave you headroom for overclocking.
I bought a 700 watt power supply. Overkill?