Well you can go up to a 1000 Watter! but i suspect that may be "Overkill" what you most likely need is a 550 watter or maybe a 650 Watt Unit. The existing 500 Watter is NO slouch though, and I wonder if that may not just do the job anyway?

Well its depending on how much wattage and amps you need. You could always get a hec 600w on newegg for $40. Another good powersupply brand is Corsair but, is more costly. Just open your computer up and see if it has a 20pin or 24 pin motherboard power connector, and then you know what kind of PSU you should get ^_^.