The biggest difference between the i5-2430M and the i7-2670QM is that all i7 with the "QM" designation are quad cores. Al mobile Core i3 and i5 CPUs are only dual cores. For games, the cache does not make much of a difference. For games that can take advantage of more than 2 cores, the performance will be better with the i7. However, most games still only use dual cores.
I would go with the i5-2430M and use the extra $100 towards a graphics card upgrade which can prolong the life of the laptop for gaming purposes.