The RAM requirements of each PC are different. It really all depends on how much software you run in background processes.
Think about a fresh install. OS, Drivers, Games, Important apps.
1GIG seems great even for BF2. Then you start installing all kinds of things. May-be some desktop enhancements, So High res. Wallpaper, media players, instant messangers, temp monitors, mouse driver apps, anti spyware, valve & other dev. downloaders and before you know it, you've got 60 to 70 processes running on your machine... lol like me.
My 2GIG is left with about 30% remaining during any newer gameplay. But, After my fresh installs, i'm at over 50% remaining.