In the PSU discussion. If there are more than one 12V rails, combine the amps for the total. The amperage rating on video cards is the recommended rating for the system, not just for the card, so splitting amperage between two rails for you system will do the trick.
And so far with Vista, to make it run perfectly, you need something like 4 GB of RAM, a huge video card, plus a whole mishmash of other things.