4gb ram is more then enough. To be honest wasting money on a pentium d system with your priority being gaming is pointless. My basic 5930g with the slowest c2d p7350 2ghz cpu and a 9600m gt gddr3 graphics card has more chance of playing games better then you.
I have 3gb ram and windowx xp currently installed and play games fine. 4gb is more then enough for any game especially with a low end 9500gt you won't need much ram as your graphics card won't play at higher resolution. Yes I am talking about system ram.
Also your motherboard supports I believe a 45nm core 2 duo cpu. Get that and you will going from a tractor your (pentium d) to a sports car (A 45nm core 2 duo).