The components of Avast are not taking up much memory. I guess you used the taskmanager to see the memory(?) The numbers there about the amount of memory that is used by a application are not acurate, as, I think it was Vlk has, explained in another thread on this board.
Games nowadays are mostly using a lot of video memory. That is not your ram. Video memory and ram are two different things. Looking at what you told us sofar, I would say it is time to upgrade your system.
And personally I would never compromise security for a game.