sell it for $150 on craigslist and gather up $80 more.
then for $280 (after selling and racking up $80 more), you can get this
COD4 on max at 1680x1050 at 40fps+, Crysis on high at 1280x1024 at 30fps. That system alone is on a completely different level than your current machine.
If you're just looking to upgrade, you're not gonna be able to do much if your board doesnt support the a dual-core processor and 4GB of memory. and if you're stuck with AGP, the highest you can go is the Radeon HD3850 (which will be bottlenecked by your P4), and the 8800GT is still way more powerful and much cheaper (the AGP 3850 is damn expensive)
EDIT: i accidentally picked the DDR2-667 2GB dimm, pick the DDR2-800 instead, it should be about the same price.