Take the Vista now and ask BestBuy for free installation of the 7.0 upgrade "at a future date." Since you're the one who's been inconvenienced, they should be willing to handle it for you, for free.
Make sure you update Vista as soon as you get the new computer hooked up, install/update the virus and adware scanners, and make a restore point/recovery disk. That way you start out on the right foot. Then scan and run updates regularly.
Most important: use an external drive for your documents, pictures, etc. That makes the upgrade to a new release infinitely easier. Plus, you can do back ups more easily.
As others have said, Vista's pretty stable right now. I upgraded six computers last week from XP to Vista with no problems. The users are happier because that's what they use at home and that's what their friends in other departments have, lol. They all still want new computers, but that's another budget battle. It's all about status, hahaha!
Yes, but a computer built for Windows 7 will be better than a computer built for Vista running Windows 7.
This is a good point - make sure the new computer meets the requirements for Win7.0. They usually have a sticker on it that says "Win xx Ready" but I'd pay careful attention to the memory requirements. Personally, I max out the memory on every new computer I buy for home or work. It just takes away one excuse for why something runs slowly, lol. (Can you tell I fight with the Helpless Desk regularly?)