This is a little after the fact, but I just did a burn on my main system, with a quad core at 4.0Ghz, a GTX280 clocked pretty high (core-777/shader-1620/mem-1404), with 10 fans and two water pumps. I used OCCT and a fur demo to try and get the system pulling as much power as possible, and used my UPS to track power consumption.
The highest consumption that I was able to obtain was 495W from the wall outlet. When you factor in the 80%-ish efficiency, thats around 400-425W actually used by the system itself.
So it's your call, but my vote says that the 620 will handle things fine.
i7 2600K | ASUS Maximus IV GENE-Z | 580GTX | Corsair DDR3-2133
OK thanks, I was hoping to avoid the purchase of a new power supply, the upgrade planned is at the max of the budget, maybe I will just get a gtx 260 instead. Then I won't have to worry about power. When I bought the 620 it was to last me many years, not 18 months LOL!
Anyone who thinks into the future with computers doesn't understand how volatile the computer market is.