Compaq presario 5000 ati radeon 4670

Hello,
I'm trying to build a cheap gaming system for my son. I bought a compaq presario with a 8 mg nvidia video card, but it is not enough to run skyrim. I have upgrded the power source to 400 watts, and want to run the ati radeon 4670. The specs on the side of the computer read-800 mhz amd duron processor 64 mb memory. 40.0 gb hard drive. 56k modem 10/1 100 mb pci nic. upgradeable agp slots. supports 2x and 4x agp cards. I am novice at best, but really need to get my son something to game on. Any help would be greatly appreciated. can you make this Dad a hero? Jeff