If I remember right an 8800 GTS 320 MB needs 26 amps, and an x1950xt needs 30 amps. They're both PCI-E, which means a new motherboard is needed. That means a new CPU (AM2 or socket 775) is needed too.
These cards need a dual-core CPU to work best anyway. Recent mobos need DDR2 RAM, so that would need to be replaced too. I think we're looking at a CPU/PSU/MB/RAM/Video card upgrade here, not just PSU
That is, you can't really make meaningful upgrades to that PC, you just need a new one.
Edit: there is an x1950xt AGP, that's as far as AGP goes AFAIK. However, it won't work so well with that CPU. Sorry