underperforming computer help

I just got a 4850 and upgraded from a 8600gts. Although it does great in source games its still not doing very well in cod4 multiplayer. Back when I built my computer i borrowed a friends 8800gt and it was the same story. His computer was identical to mine except the 8800gt and a e8400 instead of a e6750 and his computer got well over 100fps constant and I stayed around 30-40 when using his 8800gt.

I want to figure out what the hell the problem is. I see the benchmarks putting cod4 well over 60 fps on the 4850 on maxed graphics and on higher resolutions than 1280x1024 and im lagging around at 30-40 fps. Any ideas?

My setup

e6750 stock
radeon 4850
2gb ballistix ram
550w thermaltake power supply
msi 650i mobo
160gb hd
centurion 5 case
freezer 7 cpu cooler

Thanks in advance for any help!

If it's online multiplayer, its probably the latency thats causing the slowdown. If its not check that your psu is fine, ie. outputting enough power and correct voltage, and that your cpu's idle and load temps are fine. If you are using Vista, get another 2gb RAM and make sure SP1 is installed.

im using vista with sp1 and my temps are good...i also have good internet im playing with 50ms ping

i tried using my friends psu and it didnt change anything

any other ideas? 2gb more ram? i just ran crysis demo on high and it was very smooth...source runs 3 times better than before...maybe its just this game?

Maybe a format? It should cure all software faults. Did you also install the ati 48xx hotfix? Download 'driver sweeper' to remove any nvidia drivers that may cause driver conflicts if you choose not to format.