Looks like the next up grade is a new computer
PCI Express has supplanted AGP in Graphics
64bit and dual core processors give the grunt for Games
Get the drift
If you self build nut then a New MoBo/Graphics/Up the PSU should do it.
You will need to Reformat- Windoze wont run with the new MoBo-It'll 'Think' it is being cloned to another machine.[Well it is really]
Come back if you are going to buy a new machine.
Self build - try click here for one of their MoBo/processor/memory bundles good value and come all built ready to go after plugging up drives and PSU