Phædrus2401

It should run COD4 and most other games on high settings, though I'd drop the extra $50 and get a Radeon 4870 or Nvidia GTX260 and play on ultra-high/max (though if you have a really, really big monitor you might just get high with those). Using XP Home x64 SP3 instead of Vista will give you another $20 to spend, which you can use to get a Core 2 Duo E8500 instead of the E8400. If you want you can use the Windows 7 RC version (available for free until the end of June) which would give you $100 to spend, but you'd still have to pay for the Windows 7 install within a year. If you went with that option I would upgrade the PSU to a Corsair 750 watt and the GPU to a Radeon 4890 or GTX270/275.

EDIT: Buy a new motherboard unless the old one has an LGA775 socket, P45 chipset, a PCI-Express 2.0 slot, DDR2 RAM slots, and is by a respectable maker such as Asus or Gigabyte.