First, maximum supported RAM for Windows XP is 4GB's, not 3GB.
Second, making Vista a 64-bit ONLY would be DUMB. Think about it. In order to upgrade to Vista you have to ENSURE you're using a 64-bit CPU (not too bad....) AND you have to replace ALL you applications and drivers with 64-bit versions. While you may say "So what"...think of the Corporations that have THOUSANDS of machines that would suddenly need a new MS Office license..... Adoption of Vista would be slowed WAY down....
Try putting 4 gig in a 32bit windows and you will be greated with somewhere around 3 gig.
The limit of 32 bit adressing is 4 gig, but other ressources require adressing so the maximum memory you can use is around 3gig. I would assume that Vista is the same.
It does run really smooth with 2 gig, altough when multitasking(IExplore, torrents,msn,etc) it takes up to 1.25 which is a lot. When running some encoding or even some games I think I can max out the 2 gig fairly easily...
This is one thing I am trying to consider with the purchase. I was running the 64bit Vista RC1 and it is very ram hungry. I have 2 gigs now and it ran very well but I could tell it used a lot of resources. When I switched back to XP it felt like going from XP to Win98. I can definitely see in about 1 year to 18 months looking at 4gigs the way we look at 2gigs today. And that is my problem with buying the 32bit version today, it's like a bandaid. I hate buying OSs anyway and don't want to have to pay for another one in a year. I really hope driver support and apps will be more available for the 64bit version this time around versus XP 64bit.