You need a new motherboard which can support a modern GPU which will require you to move to atleast DDR2 RAM and a new CPU. If you have about 400 to spend on the upgrade you can get a decent system out of it but the only parts you should keep are your optical drive, hard drive, and maybe your case.
I suggest if you are flat broke to just buy a new board that does feature at least a agp slot while a few but very rare 478 boards do feature a pci-e slot but regardless you are going to get very poor performance. If you got the pockets its time to upgrade.
What games do you have installed...
A quick look at Ebay will find a AGP GPU that will play games (The lowest common denominator in the setup)
What PSU do you have (Take it generic 250w or poss 300w)
(still have a p4 system with agp 1950pro, can still game fairly decent, except modern multi core games)
I think you could get a bit of low res gaming done with a few bob spent, like $100 or so,but is it not better to take that money and spend it on a better modern system!
Buddy, sorry for the bad news, but you ain't gonna be able to play the above games on your current setup! The processor "MAY" be able to struggle through, but you GPU and RAM definitely need to be upgraded....which means upgrading the bobo as well! Oh, and don't forget, a new PSU to power the GPU. Your RAM will have to be minimum DDR2 (3 recommended). GPU, at least the nVidia 9800GT/ Radeon 5750
^+1 You are not going to play those games with any amount of upgrades with current processor.
Now if you had said some older games (Far cry, prey, call of juarez, fear 1st one, stalker, 1st one heck even oblivian..), then yes possibly, but modern games need a modern system.. sorry!