Also make sure that you have set up the correct connection speed in the steam settings, I heard that it can make quite a difference to your latency ingame. Other than that, reduce the resolution and quality settings.
Im not pushing my resolution down it will make it smoothe for the whole computer but not for me . My current resolution is at 1024 x 768 and my specs are
6600GT AGP
1GB GeIL Memory
AMD 64 3000+ <Winchester>
Gigabyte K8NS Ultra 939
1.5mbps down or 150kb/s ADSL
..btw I built this comp yesterday.
I get good fps about 50 - 80 constant in multiplayer it just like stutters in a graphical fashion. Thanks