The CPU is decent enough for some gaming. I had a E4600 Overclocked from 2.4 to 3.0 and it ran almost all my games (Minus Crysis 3) Great. Only have 4-6 Gigs of ram at the time and a GTX 260. Now i got 8 Gigs DDR2 800 Core 2 Quad Q9550 2.83 OCed to 3.5 and a GTX 480 and i can play Crysis 3 on medium at 1920x1200 no issues.
now most video cards require at least 500 watts. now its not just the PSU though. It also depends on other things like if you have a bunch of hard drives, other addin cards ect. But for the most part everything minus the GPU usually don't take up more than 200 Watts or so.
I would get a better PSU though. Get like a nice 750W that you can use on future builds, and then get a nice video card. Are you looking to get the 650 Ti or replace it?