ATI 9800 Pro Doom 3 performance

[PickledSheep]

Hello. Just like many i have just bought doom 3. I saw reports of great performance with doom 3. I will post some benchies tomorrow, but notice is running awful on my system. Constant chug. I am run a ATI 9800 Pro, with the 4.9b catalysts. My specs:
Barton 2500+ (am i CPU limited?)
512mb XMS corsair mem
a7n8x-DX mobo
2 WD800JB in stripe
ect ect ect
any ideas?