Why would you want to do that? You can use 4 gigs in and xp machine.
The only difference between vista and xp is that with vista you need at least 3 gigs with xp 2 is sufficient, in my opinion.
A 32 bit Vista doesn't see any more memory than a 32bit XP does. You can put in 4 gigs but it won't see much more than 3 gigs of that. 64 bit OS would, but Geovision doesn't support them.
So, stick with XP.