This got me wondering as i run a simular system to you with 4GB Ram.
I played for an hour today and monitored the usage in task manager.
It never peaked above 1.5GB
I was playing on MAXIMUM Graphics 1680X1050 settings FULL AA & Effects etc with Cinema Surround Sound selected.
Average Usage: (Image Missed out a 0)
1032290 K Which is what? About 1GB
I monitord the usage on my second monitor during play.