**Vrugz version at the bottom of post**
You will definitely need new ram.
Your current board is using DDR 400 PC 3200 and the one you mentioned upgrading to most likely takes up to DDR2 1066 PC2 8500 sticks.
The board's specs should say somewhere if it takes up to DDR2 800 or DDR2 1066.
Ram is fairly cheap now so you can get a decent amount of either for a reasonable price.
As for Vista.
Let me first preface this by saying that I am *far* from a Microsoft fanboy.
I'm currently running Vista Ultimate 64bit but was dual booting between Vista and XP for the previous year.
Vista has actually come a long way. It does take a little getting used to but Service pack 1 has really cleaned up a lot of issues and I've been very pleased with it since.
In your case I would say it really depends on a couple of things.
If you plan on having more than 4 gig of ram XP 64 or Vista will be a must over Regular XP.
If Vista comes with your new rig for free. Awesome. Put service pack 1 on it, turn off the security prompts and you'll be fairly impressed.
If you already have an XP CD and don't plan on having more than 4 gig of ram, I'd say XP all the way. Spending the cash for a Vista CD if you already have an XP cd isn't really worth it for the marginal performance enhancements.
As for all of your other previous hardware. More than likely everything else can be reused without issue.
Your new motherboard should come with plenty of extra cables as well.
If you want some tips on backing up or moving WoW let me know.
The 600W power supply should be fine for running the 8800GT and your new processor.
If you think you might drop a second 8800GT in for SLI later on, you may need to up the power supply a notch as they each take about 110W.
If you're thinking about going all out from the 8800's to the 9800's. I'd definitely recommend a bigger power supply as those beasts eat between 225W-300W a piece under load. http://educations.newegg.com/tool/psucalc/index.html
600W Power supply is fine. Maybe take it a step up if you plan on running two 8800's sli though.
You will need new ram. Check which is the maximum speed your new board will support and buy that.
2 gig is fine, 4 gig is better, 8 gig if its going on the company card.
If you have an XP CD already use that. If Vista comes with your new rig for free use that instead.