I used to have a similar PC and it couldn't even run the youtube videos of Crysis 2.....
You need a new PC. I'd be willing to bet that motherboard is still stuck on AGP which is outdated and no longer used by video card manufacturers. Plus if it is a P4 it is either socket 478 or 775 which are both outdated and you're not going to be able to find a modern CPU to replace it.
Do yourself a favor and start thinking about a complete new build, you can save the case and CD/DVD drives, even the hard drive if you want, but you'll need a new mobo, new ram, new CPU, new video card, and a copy of Windows 7 because it would be a sin to build a new PC and stick with XP!