you'll need 64bit Vista for even 4gb of memory. about 1gb goes to PCI addressing in a 32 bit OS, leaving you with 3gb. so keep that in mind for driver support when selecting devices and software..
I have 4GB on WinXP and i "see" around 3.5GB -- although that may vary a bit from computer to computer. People generally get somewhere between 3-3.6GB if 4GBs are installed under 32bit OS.
But for users that dont multitask alot...and dont do any intensive graphic stuff...2GB is already enough...and they wont see any real measurable difference between 2GB and 4GB.
