Battlefield Vietnam doesn't have the fastest performing engine.
I have noticed that with every game in the Battlefield series. Anyway, I am running Battlefield Vietnam in the highest settings, and I am getting great preformance, and im using a standard stock 6800. I getting 90 and up for FPS both online, and on created games. First off, what kind of internet connection are you running? Also, when your in a laggy server online, check the ping, if your over 120 or more, your going to tend to slow down. But if your getting low pings, what I would probably recommend, not that this is the thing slowing you down but, upgrade the CPU, go with something AMD, I would recommend, if your short on cash pick up a Socket 754 Athlon 64 3000+ or 3200+, but if your the type who makes upgrades on vital parts often, get the Socket 939 chip in those series. Reason I say socket 754, is because by the time most people make a change in the CPU, they usually change the motherboard at the same time. Also, if your not strapped for cash, get another 512 stick of Ram. As Mission said, if your using a low wattage psu, you could be underpowered, even if you think a 350 Watt PSU is enough, its always good to be on the safe side and go for a much higher wattage.