If you are on Vista 32 bit and you are tring to upgrade to 7 64 bit, you have to do a clean install (format the drive). Not sure about Win 7, but my Vista Ultimate upgrade had two different DVD's; one for 32 bit and one for 64 bit. If you are on Vista 64 going to Win 7 64, you can start setup from within Windows. The recommended setup for OS upgrades is to always do a clean install.
Backup all of your data, set the CD to be the first boot device, and follow the instructions.
I think it doesn't matter whether you have 32 bit vista or 64 bit vista. Load the 64 bit DVD from within windows and choose a clean install. But, be sure that you have all of the 64 bit device drivers before you start the install. At the very least you need the driver for your lan device so that you can connect to the web. Also, go to the Window 7 website and download the easy transfer utility. I will save you a ton of headaches. It saves all of you data files and windows settings such a mail files, etc.
I just finished upgrading from XP and I used the information on the Win 7 upgrade website to guide me through the upgrade. I was amazed that it even retained my dual boot configuration into Linux. M$ finally did it right.