Check if the GPU fan still works, if it doesn´t replace it, if it does, clean the fan and remove excessive dust from the rest of the card as well. If both doesn´t help try underclocking the GPU and/or the video RAM with ATI Tray Tools, that should be a cheap workaround:
http://www.guru3d.com/article/atitraytools/189/
If that also doesn´t help get a cheap AGP replacement card on ebay or a local computer shop. A used 9700 or 9800 probably doesn´t cost more than $30.