If you can find an old 6600gt, that will be much better than GF3. What about a Radeon 1650pro, that should be able to play COD4 ok and also not be too expensive. Nvidia dont seem to be interested in AGP anymore.

I had X2 4600 skt 939, 2GB DDR1 RAM and intially Geforce 7900GTX.

3DMark 2001 score: 25000

3DMark 2006 score: 5700

Aquamark score: 75000

In an attempt to improve performance I upgraded my Gfx to an ATI 4850.

Whilst this did improve my scores, the results were disappointing.

3DMark 2001 score: 29000

3DMark 2006 score: 8500

Aquamark score: 85000

I knew then that to improve performance significantly would require an architecturel change. The X2 4600 was bottlenecking the whole system. So, I got an E8400, 2GB DDR2 and a Gigabyte P45 DS3 board.

The scores went up far beyond my expectations

3DMark 2001 score: 58000

3DMark 2006 score: 12500

Aquamark score: 175000

I reduced the E8400 clock speed from 3GHz to 2GHz to see what impact this would have and also to see if it was only clock speed that was the only factor for the slow Athlon performance.

3DMark 2001 score: 40000

3DMark 2006 score: 10500

Aquamark score: 122000

Whilst clock speed does have a significant influence, it appears other factors such as processor and motherboard architecture also have some bearing. My conclusion is that going from an Athlon X2 to an E8400, especially if its socket 939, will show amazing improvements in performance.