need advice on which computer is better?

I have a buddy who is selling his HP pavilion computer (CHEAP!) which has intel pent g640 dual core with 2.8ghz and 6gb (upgradable to 32gb) of ram and if I buy it I plan on putting a 500w power supply and of course a better vid card and the computer I have now that I wanna replace with the HP is a no name brand intel quad core Q6600 2.4 4gb ram (MAX) and I wanna be able to game on it like I can...