A 8400GS may run it but it'll be very poor. When running games of this nature, the graphics card (GPU) is the most important component - next come CPU & RAM. In addition, the speed of the GPU's RAM rather than the amount (unless you have a very large monitor) is what to look out for.
I think you'll need something like a 7600GS or better for optimum performance. A few to choose from....
Least to go for - Innovision GeForce 7600GS 512MB DDR2 PCIE click here (read one of the long reviews lower down click here) = £38.45 excl del (free del if order over £49.99)
Recommended - for only £5-6 more, the significantly better Nvidia GeForce 7600GT 256MB DDR3 PCIE click here = £43.99 excl del (free del if order over £49.99)
If you can stump up a bit more, this Nvidia 7800GTX 256MB DDR3 PCI-E click here for just under £60 delivered is very good for the money. It's on par with a ATi X1950 Pro that goes for a bit more. But if it's only for COD4, stick with one of the 2 above.
Are you going to be playing any other games? If yes, which ones? G
P.S. You can compare performance between various GPUs at click here
Not really. I have a 7600GS and its borderline for COD4 as I've tried the demo. I had to turn a lot of the setting down or off to get it to run ok. I must say that even then it still looked good. Assumming its PCI Express you are talking about and not AGP I would be looking for at least a 7600GT and I would check it came with DDR3 memory as some have DDR2 memory and are not quite as fast. Or a 8600GT you can get that a reasonable price. I should point out maybe that with high graphics hungry games you will still have to lower some settings, to play some games at full settings at a high resolution you are talking about a lot of money. My 7600GS is ok but as I said its borderline. Personally I would be looking for something like a 8600GT with 512mb RAM. That one Crossbow7 linked to the 7800GTX looks interesting especially for the price.