If you are going to buy a new OS for your new pc, get Windows 7 it's a much improved OS. That's not to say that Vista is no good, with all the updates and Service pack it's fine.
I used it for a couple of years without a problem.
There is no point in buying an OS that's been replaced, get W7 and you won't be disappointed. It won't take you long to get used to it.
Another thing to consider, if you get the Business or Ultimate version, you can install XP on it to run older programs that are not compatible with W7. You run XP in W7, so no dual boot required.
You can go to the Windows 7 Upgrade Advisor page and check out features.http://windows.microsoft.com/en-us/windows/downloads/upgrade-advisor