When you buy Vista, you get the x32 version. Then, using that license, you can order the x64 disc for another 90 Skr (~EUR 10).
I got the new disc a couple of days ago and so I installed Vista x64.
This is what I found out:
-x64 is faster
-x64 is very stable
-x64 handles 4GB RAM
-x64 runs most x86 programs
-x64 runs IL2
-x64 is what you should get if you want Vista. Provided that you have suitable hardware, I see no reason at all to stay with the 32 bits version.
I will install Vista x64 on my gaming rig too, but I am still waiting for Saitek to finish their Vista SST-drivers, hopefully with less bugs than the XP-version.