Vista vs XP Performance? What should I do?

In a word, NO! At least for Vista x64 SP1 was a complete disaster, creates more problems than it solves and still is 10-50% slower in some DX9 titles.

Not sure about Vista x86 (32bit).

Get Vista if there is a DX10 game you MUST have and MUST play under DX10 API. I have ALL the DX10 game titles for Vista and not a single one really looks significantly better than the DX9c version. UAC in Vista is horrible BTW, turn it off immediately if you get Vista. Vista is VERY buggy with older (as in 1-2 years old) DX9 titles.

Again this is all with Vista x64, don't have any experience with Vista x86.

My hope is Microsoft actually start to listen to what consumers want -- most likely a fantasy. But the sooner Vista is replaced by something else (anything else), the better for all.

SP1 is definitely NOT the what I was hoping it would be.

if you are bulding a new rig than install vista it will take advantage of all your 4 gb of memory and in the future when you upgrade to a dx10 card you will have a dx10 you wont need to install new os

Actually Vista 64 bit will take advantage of 4 gigs, 32 bit wont, why not dual boot? You do know you can have xp running on 3 personal home computers without breaking any contracts right? Also vista sp1 is yet to be seen, people had issues with a incremental update, and ms pulled it until next month, and then you can d/l sp1.