If you play games like quake 4, bf2/2142, oblivion, etc. and like to ramp the detail settings to high or max you will get much smoother gameplay with 2gig of ram (in some cases they would be unplayable with just 1gig), if you only play on medium or lower settings however then you probably won't see a huger difference...
I highly reccomend 2gig however as I find it gives me a smoother overall experience and decent headroom even just in windows.