For me, 3GB is the point at which I begin to 'waste' memory in everyday normal usage (i.e. no database or CAD packages running). I simply cannot get Windows 7 to use any more - I am using just under 1GB with Windows and whatever applications I am running, and it has cached just under 2GB in Superfetch and now it seems that there is nothing else left to cache. With 2GB physical RAM, Windows 7 would fill up as much as it could, now with 6GB (3.25GB available) I still have 500MB free.
Of course, I am currently running Windows 7 32-bit (going to upgrade to 64-bit this weekend hopefully) and so after moving to 64-bit the memory usage will be a bit higher anyway. In that case, 4GB seems to be the sweet spot. I wonder how much is going to be wasted once I have 6GB to play with. It will be nice to have when SolidWorks is running but other than that...