^5 +1 what ct1615 and moody89 said!
Your best bang for the buck is to upgrade to an ATI Radeon HD5770 video card and Windows 7.
The 5770 is an energy efficient video card.
Here are the official ATI recommended power supply requirements for the ATI Radeon HD 5770 and 5750 video cards:
450 Watt or greater power supply with one 75 watt, 6-pin PCI Express® power connector recommended
600 Watt or greater power supply with two 75 watt, 6-pin PCI Express® power connectors for ATI CrossFireX™ technology in dual mode.
The recommendations are for an entire pc system..