I'll assume you're using a 64-bit OS, because if not, you won't use more than 3.75GB, regardless of your mobo's limitation. Generally, gaming isn't very demanding on RAM. You really won't need more than 4GB (2GB, really, but if you're rocking Windows Vista or 7, the mininum for 64-bit is 2GB; so add in 2GB for gaming and you get 4GB.
What are your system specs?