Sigh... the actual question has nothing to do with 32/64 or xp vs vista. He's asking why Vista says he has 4gb yet task manager clearly says it's only seeing 2.8gb
Starting with Vista, Microsoft decided to show "installed" memory vs "usable" memory. Let's not debate on the merits of this change, but the fact remains that people with 4gb installed, Vista will now say you have 4gb installed, regardless of how much the OS can actually use after the motherboard, SLI, take their piece along with limitations of 32bit.
Just an FYI, starting with SP3, Windows XP will do the same now.
I suppose this will let non-technical people who actually bought 4gb of memory for their 32bit OS to stay in stupid-land thinking they are using it all. I'm just waiting for the lawsuits...