Replacing the cpu with an Athlon FX might squeeze a little extra speed out of it, but it's probably going to cost a lot and deliver not much.
Increasing the ram to 2 gb might help a little bit, and that's a cheaper option.
The weakest part of your system as far as these top end games are concerned is the graphics card. It's now really a mid range card and doesn't support shader 3 technology.
Your mobo has only 1 PCI-e x16 slot, so bung in the best card you can afford. The X1900XTX is currently the top dog graphics wise.
Just make sure your psu will power it.
Upgrading beyond that, and I think you'd get best value by buying a new gaming pc. Two small HDD's in RAID 0 configuration, faster memory, whatever's the latest CPU, overclocked and water cooled, two top-end grtaphics cards in SLI or crossfire mode, and not much change from 2 grand.
I'm still gaming on a 4 year old Athlon XP 2100+ and have only recently upgraded the graphics to a plain vanilla 6600. I blame the wife, dog and 2 teenagers I have to support