Well, I had 4GB of RAM in 32bit XP, didn't work. My mate said 32bit Vista doesn't do it either. I'm running 64bit Vista and it picks it all up. There's no real advantage to 64bit operating system in my opinion, in fact it's more of a disadvantage because a lot of software and drivers are created for 32bit operating systems.
4GB of RAM works fine in XP/Vista 32-bit, but only 3.5GB of it will be detected. If you use XP especially, 3.5GB is plenty. Vista it might get a bit tight, so it's better to use 64-bit Vista than 32-bit. The driver issues aren't too bad with 64-bit any more. nVidia's video drivers for vista are still relatively mediocre, but they do work. For the record, 64-bit supporting applications work very well in vista-64. WinRAR is incredibly fast in Vista for me.
