If you're going to OC, then the Q9400 for its R0 stepping. If you're not going to OC, then the Q9450 for its larger cache. IMO, if you want a Core 2 Quad, the Q6600 is the best value if you OC, and then after that the Q9550.
You sure you need a quad for gaming? If you want good value for gaming and can OC high, then why not consider the E7500 or E7400 (both with R0 stepping)?