To address over 4GB RAM, you need not only a 64-bit process OS but also 64-bit software.
Most games are 32-bit processes anyway, so they won't benefit from having more RAM available. If you use your windows partition purely for gaming, the game will be the only application running. Thus, even if you use a 64-bit OS, the 32-bit game will see only little to none benefit, because its memory limit of around 2GB stays. So the 12GM RAM will be unused anyway.
In time, we will see more demanding games, which will be 64-bit processes and will need more RAM. Still, more RAM usage means more hi-res textures and high-res models (other components of a competently programmed game do not have high RAM requirements). I have a strong suspicion that you will sooner run into the 650M GPU being a bottleneck with these future games than the RAM though. Most games stream data from the disk to the GPU, using RAM as an intermediate buffer, the ultra-fast SSD on the MBP will work charms in this situation.
To sum it up: it may be that you will win few percents of performance by using a 64-bit OS, but nothing substantial until the games become true 64-bit application with high RAM requirements. Til then, the additional disk space used by the 64bit windows is basically wasted. Please note that this is all under assumption that the Windows partition is used for gaming only - clearly, if you use it differently, 64-bit Windows might be a better option.