I have an HP Media Center PC for about 2 years now equipped with a Radeon X300 128mb card. I am in need for something 256 but my PSU is only about 300W. I can't upgrade the PSU right because I am still paying this one off and my new gamer rig would be possible for another year or so. My game choices are slim because of the card and PSU. Anybody suggest a card I can get until I can do better that would work with my PSU. Please I am in dire need. I can't stand looking at Age of Empires III this way.
I don't think you'll be able to go crazy with the video cards (no 8800s for you!) but I'm pretty sure you'd be fine with something like a 1650 (if you're an ATI fan) or perhaps a 7600 GT (if you're an Nvidia fan)
Why not consult the power calculator? Here's one I found...
I believe there may be a better one out there, but hopefully this will get you going in the right direction.
EITHER of those cards would be a dramatic improvement over your current card and neither would break the bank. I'm not sure if your particular model has a PCI-E slot so make sure you know what type you've got before you run out and buy a card.