I used to have a Q6600, 7900gt running on a stock standard PSU (550w) that came with case, worked fine TILL I upgraded to a 8800GT, only sometimes would the machine boot, if it did it would only last a few minutes till stressed then shut down. I replaced it with a (then) Corsair HX520, never had a prob, now I use it on a i5-3550, with a GTX 560, still going very strong, and never had a prob with it, so yeah get a decent PSU mate.