I run Mac OS on a MacBook Pro with 4GB of ram. I use windows at work a lot and I think windows 7 really benefits from running a 64bit OS, the more ram the better. Its a sluggish operating system anyway. If you run more then 4GB on a mac in 64bit on Snow Leopard and use any graphic program or logic pro you will really see a big improvement. On games, they are not designed to work in 64bit, so anything over 4GB would not even be used. Not yet anyway, give it a year tops.
I have 4GB and I havent run into any problems yet. But I dont play any FPS or stuff like that. Id think a lot of the RAM would be wasted because IIRC, 32-bit programs cant see more than 4gb.
they can't see more than 2GB unless they are flagged "/LARGEADDRESSAWARE" then they can use ~3GB on 32 bit systems and ~4GB on 64 bit systems