Have 32bit want 64bit

You can't 'upgrade'. You have to install a 64 bit Operating system.

If you have an existing 32 bit install, then the easiest thing to do is grab another hard drive, install Vista 64 to that, and configure for dual boot. You'll keep all of your existing data and programs, plus your 64 bit install will be able to see/use data that exists on the 32 bit driver (and vice versa).

You'll need to reach out and grab the necessary 64 bit drivers for your 64 bit install. 32 bit ones won't work. Microsoft have made it a requirement that any maker who wants a Windows logo on the box must provide fully functional 64 bit drivers. Same with games - If you want a windows logo, your game has to work on both. So this shouldn't be a problem.

The limitations are that when you upgrade your memory, your 32 bit install won't be able to use anything more than it already can. And any games or programs you have on your 32 bit drive will likely need to be reinstalled on the 64 bit one.