I am not saying it is a problem specific to COD5, it's just a good example to illustrate my point here.

What makes me difficult to understand is that both COD4 and COD5 are using the same graphics engine, but the frame rates I am getting from both games are different, given the same graphics settings. What I have noticed is that in COD4, I can manage to obtain around 60 fps running my Q6600 @ stock speed which is 2.4GHz without overclocking it. But then fps bleeds bitterly when running COD5 which givesme somewhere around 30 ~32.

I am actually amazed at how my HD 3850 performs having said all that. After migration from my prvious system which was only a Pentium D945 with ATi X1950Pro (AGP version) to my current rig which is a C2Q Q6600 with a PCI-Express HD 3850, the following games run pretty good with frame rates capped at 100 fps (no AA, no AF):

Battlefield 2
Battlefield 2142

Even talking about AA and AF applied, I can still manage to get 77~80 fps for both games with BF 2142 slightly lower.

My question being, is the Quad CPU at stock speed already suffices the need for good performance, or the HD 3850 GPU is contributing to the high fps so obtained?

Are leafs hurting the fps, COD5 is more outdoors with lots of jungle scene, whereas COD4 is pretty much like indoors with less leafs to deal with thus making HD 3850 stands out?

easiest way to upgrade any comp is the gpu. My suggestion would be a mild OC of your cpu to 2.8-3ghz and replace the gpu with a 4850. You will see no improvement in any of the BF2 titles (the human eye cannot detect anything faster then 40fps give or take a few fps) but you should see a substantial improvement in cod4/5.

FYI, I was getting 100fps in all bf2 titles with a P4 3ghz and an 8800gts so that title is definately tied more to the gpu then the cpu. When I upgraded the cpu I started getting much better fps in cod4/5

Many thanks for this information! It has been a bit difficult for me to confirm this fact because I upgraded both the CPU and GPU altogether. Despite my previous CPU was a Pentium D 945 running at 3.4Ghz(w/o o/c) and previous GPU was Sapphire X1950Pro (AGP) 512MB, the CPU was pretty fast, but now I realise it is the X1950Pro is the bottle neck.

Im having the same problem with COD:WAW. in COD4 i get125+fps. in WAW i get a measily 20 or so. same settings, with a 8400@3.6 and a 4870