I realize this question has been asked but I think I might be the only one who will tell you exactly what I will be running.
A teamspeak program (skype would be the biggest one I imagine)
A game. (just pick the most intensive RAM game you know to be on the safe side)
If I ever need firefox open, I don't really feel to bad needing to write something down then closing it. (you know with pen and paper like they did in the old days =P ) If I need some info on a game or something.
I might run GameBooster - but that would be increasing available RAM not decreasing it I imagine.
If it matters I will be using the gtx 560 ti 448 and an i5 2300.
4gigs is enough for almost anyone. The only times you'd need more is if you're running VMs or doing audio recording/video editing on a high level scale. I run 4 gigs in my build and I game more often than not, never see my ram usage go past 60%.
4GB will be sufficient. More RAM will only become a necessity if you do heavy multitasking. What you describe does not fall in that category. Game code doesn't need as much system RAM as you would imagine. And the heavy graphical work is done by the video card and stored in its fast on-board memory.