Well what game is it? If it is anything with the HL2 engine, its because that engine was designed for ATi.
And how are you using it. Because if you use one card with one computer in your sig, and the other card with the other computer, then your gonna get different rates.
Well something is being bottlenecked somewhere. What it looks like is the ram is bottlenecking your card. Because with mt P4 630 I am getting not-as-high-as-expected FPS. I should be getting an average of 120 (the cards full potential) but im getting around 80 and sometimes less.