Xfx 250 gts having problem

I recently bought an XFX 250 GTS core edition graphics card for my Dell Dimension 9100. A few days before the purchase, i upgraded my power supply from the stock 375 watt to a 500 watt. However, whenever i play games like WoW and call of duty WaW, the game often freezes up, and says "Hardware Failure" and then requires me to shut the computer off and turn it back on. Everything in my computer is stock, except for the power supply and graphics card. Anyone know why this is happening to my computer?