I was going to buy a 8800 GTX, but I couldn't see myself spending $500+ and the price looked like it was never going to drop so I bought the 2900 instead. I've had it for 4 days now and I haven't had any problems at all. I have 9 hard drives in my case so I was a little concerned about power at first, but after doing the actual math on what would be required I was just worrying about nothing.
Anway, to answer your questions:
620 watt corsair
I haven't noticed anything strange, seems like my X1900XT so I'd say yeah.
No, everything I've read online is an exaggeration. Its not like the stock X1900XT fan or something. I could hear that over everything until I replaced it with a Thermalright HR-03. The only thing I ever hear now is the crossflow fan in my Stacker case so everything else is quieter than that.
Don't know about XP, but everything seems fine to me in Vista. No crashes in any games or Windows yet.