You'll be supprised at what you can get away with. Although some cards say 500watt needed....in actual fact they'd proabably run on a GOOD QUALITY 400-450watt PSU.
BUT....i would be wary of your PSU as i assume it came with the pre-built PC and tend to be of poor quality.
Im not sayin go and get a power hog GPU....but i think you'll find your options arn't as limited as you might think!
^+1 for hd 4670.
(btw. Geforce 9500 wasn't designed with games in mind!!!)