like the above post said if your in a really big fire fight or what have you, and your on a 64 bit os and say that system has like 4 gig in it the simulation can take up to 4 gig of ram or what ever that is not being used up to 4 gig, but if your on 32 bit os that as far I know can only use 3 gig of ram your performance may lack a little, but I already notice difference on load times, but thats just me, :D depends on situation, MORE POWER TO 64 BIT OSes.