Running games slowly (power supply problem?)

hi, i've recently installed Witcher 2 and there's one moment in the story (where the dragon flies overand sets the surrounding scene on fire) which causes the fps to plummet below 15.

i checked the EVGA precision tool and it says that the gpu is at 100% usage which i found to be wierd as i'm running a gtx460 and the recommended is a gtx260. CPU usage was floating at around 80% (for the highest core) and around 60% (for the second highest core).

so my guess is, is it my PSU problem? i know that it is a very poor PSU but i never had any problems with previous games. (it's a Storm 600W with 24A on the 12V rail).
Also, could someone point out a good PSU that i could use for this? (with room for overclocking)

i've got a phenom II X6 (OC'd to 3ghz, being cautious not to overwork my PSU)
GTX 460
4gb RAM DDR2
GA M720 US3 mobo
windows 7 32 bit.

also, should i revert my phenom II X6 back to the athlon X2 7850, or do they both have the same power consumption?