very good computer but still lag, help

[soursmell]

Mobo- x58 sabertooh
ram- corsair dominator 3x 2gb
Graphics card- 6870HD crossfired
processor- i7 950 quadcore

recently bought, my friend says i should be able to play this on maxed settings no prob AA set to 4x
but i starting lagging once i put this on ultra with both me and the pc having a full army.

Can anyone tell me if its a software issue or maybe something else?? I have the latest drivers/updates
for the computer and the graphics card. I don't think theres anything wrong with the computer hardware,
I get around 100-200 fps on Counter strike source and probably over 80 on left 4 dead2 maxed out.

[Dewy]

I assume your talking about a TW game? With that set up you shouldn't be lagging. Only 100-200 on CSS that sounds low compared to my 5870 on max settings, are you sure you have a crossfired 6870?

[mrcrusty]

What are your temperatures? What's your PSU?