For me it DOES make a difference, but it truly depnds on what sort of apps you are running. a few games benefit...BF2/2142, Bioshock, DiRT, Vegas, and a few others benefit from teh extra ram, as will many more relased in the upcoming months.
PLEASE PLEASE PLeASE do not take dude's advice and just toss in one more stick...if you want to jsut add 1gb(for the total of 3gb, which is what you get usable if you have dual vidcards, etc...) please get 2x512MB sticks so that each channel of the memory controller is evenly balanced....youčll thank yourself later.
While you may think the 4gb is helping a lot of games have a 2gb limit and will crash if they go over that and that is a common thing in Supreme Commander unless you apply the patch.
This is windows virtual addressing @ fault, not the game. Some games have a a workaround, as is applied in the like 4th or 5th SupCom patch. It really depends on what settings you are runnning in a given app that dictates it's memory usage, more often than not(high res textures get fetched by cpu for vidcard), so it really depends on what you've got and what you do for proper consideration of this problem.