Hd 4650 + my psu = boom?

you can try it, it wont blow up your system until you load up any game. Games can run fine but as soon as the cpu and gpu are at their peak performance. You're screwed. I've currently got a 9600GSO in my system on a 250w PSU and i'm afraid to turn it on since all my games freeze and i have to do a manual reset. I have a feeling it'll die out if i try anything intensive on it. don't do it man. trust me.. its not worth it. Save up for a XFX Pro Series 450w power supply like i am. DO NOT throw that card in there. It might be low power but the risk is not worth it. Your call though

I'd imagine it'll work ok. The HD4650 is a very weak card, only draws 55W or so.

It's certainly not ideal, at some point you are probably going to want to look at a proper PSU if you are buying any kind of modern card.

tell me about will it run heavy games?

Very unlikely, but it really depends on the game.