Ok I used an online PSU calculator and taking into account 4 sticks of ram 20% capacitor aging 2 hard drives 5 usb devices your CPU and either the 4850 or the 9800gtx you're good either way. 342 watts recommened.
Questions now - how much you wanna spend and whether or not you have decent airflow into your case - if not get a dual slot graphics card, if so the bang for buck of the 4850 is where you want to be IMO. xD
Tease.