I tend to keep a running backup of everything I care about so that I can reformat at pretty much a moment's notice. Backup my bookmarks and current game saves and then BOOM, done.

Thanks, I will :-)

EDIT: Unfortunately, my job derailed my plans to perform the operation this evening. Hopefully I'll get it done tomorrow. I'll post an update with the results (if my machine still works then).

It works! IT WOOOOOOOOOORKS! Finally I've invented something that works!

As you can probably tell, I succeeded in my quest. I can now boot to either system, and they both work perfectly.

Thanks for all your input. I knew I could count on my fellow GOGers :-)

Well, I did as I had planned, and as hampsterstyle outlined. Unplugged the Vista drive and put the new drive in. Installed Win7 on the new drive. Plugged the Vista drive back in and made sure the default boot drive in the BIOS was the Win7 drive. Then I installed EasyBCD (as KingofGnG suggested) on the Win7 drive and added both Win7 and Vista to it. So now I can dual boot, and can get rid of the Vista installation when I choose. But for now, it's nice to have, as I can move stuff in my own tempo, and if I run into any compatibility problems (for instance with GOG games that don't work on Win7 yet) I still have the Vista installation.