well, I've got my most important games working now, and steam was dead easy to get going! just copied over my old files and double-clicked steam.exe, and it worked!!! UFO Aftershock took ages to get going, partly due to Vista 64 issues, and partly due to sticking in a new graphics card. I've got the core hardware going, but still a few more drivers to add. I think V64 is definitely the way to go.
but then again, if I didn't have to rebuild, I'd be using XP 32 quite happily still!!!