There's no upgrade path from 32 to 64, so you'll be doing an OS installation from scratch, effectively.

With that in mind, when I did mine a few weeks ago, I opted to format the hard disk as part of the process, being careful to take backups of everything of course, and making sure I had all the 64bit drivers available on USB and network drives "just in case". I don't know how pretty an "overlay" installation of 64 would be, but I bet it wouldn't be nice...

FWIW, my installation was exceptionally painLESS and I've had no problems at all with Vista64. I've never looked back. My machine's pretty new though, and had all mainstream components.

Keyclone works fine, and once you've installed it on your shiney new 64 bit system just PM Keyclone here on the forums and he'll make reassign your licence to the new machine for free.

Good luck!