I use Vista x64 on my primary development machine. It runs everything from Visual Studio 6.0 through 2008 with no trouble.
It's not a giant leap forward, but I prefer it to XP.
If you ever need XP for anything, Virtual PC is free and works well. I keep an XP VPC image handy for testing in XP, IIS6, and IE6. The best of both worlds.
Of course, you could always get Windows Mojave instead!
In response to the question edit: I used Vista x86 for about 6 months before switching to x64. Same opinion.
Just keep in mind that you'll be wasting 0.5-1.0gb of your RAM with x86. You should be able to get nearly free x64 media here. Your license is good for both editions.
