Honestly, people who use vista 64 SOLELY in order to have 4 gigs of ram are.. stupid.
If you put 4 gigs of memory into your XP system, it will recognise around 3.3 to 3.5 gigs maybe, give or take a little (depends on various things including graphics card).
XP's base memory requirement is 128MB (or 64/96mb to push it), yet vista's is 1GB. Look at how much vista uses, just showing your goddamn desktop or to open notepad. It's insane.
So basically, any extra memory you get by having the extra from 3.3-3.5Gb in XP to 4GB in Vista is already stolen by the shitty bloated OS you're using. You have LESS memory available to you.
Vista 64 in order to use 4 gigs memory = ignorant/idiot.
If you're going to use that OS for more memory, at least use MORE than 4 gigs =x. But this is ignoring that even 2 gigs is more than enough for the vast majority of anything out there and more about bragging for your e-penis than for practicality.
There's a reason they're already hard at work on Vista's replacement, the same reason so very many people either choose to stay on XP, upgrade back to XP from vista, or insist on buying XP instead of vista
At least MS had the sense to listen to their customer base and increase the lifetime of XP in their product line till 2014.