RAM really only has an impact when there isn't enough of it. Once you have more Memory than your system uses, it ceases to be a bottleneck or a contributor. If you were moving from 3 to 6GB, there could be some noticable difference under certain circumstances - such as going back and forth between two maps - if the situation were that the increases memory enabled your computer to leave more maps actively cached. But since you're talking 6GB to 12 GB, which is more than most games in their entirety, then then answer is "Little to No Difference".
when it comes to games - the real issuse is having ram, CPU and GPU run optimally ... increase in ram wont help you except for seeing a lightning fast load time for your profiles (provided you have the right ram timmings!)