ddaydreams,

You are on the right track, your reasoning is sound. Two hard disks is the best way. But as you in any case need a backup:

1. Do an backup image of your current system on C (with for instance Acronis the best IMO)
2. Install Windows 7 on the second disk as C. (NOT dual boot)
3. Restore your old system on the first hard disk.

In bios you set the hard disk you want to have as default.
If you want to start on with other disk you just press F11 at start (or whatever your system use)

In that way both systems will be "as C" Vista and Win 7 automatically rename the "not used" C. And when you get a new computer you can just copy over your backups (with the 100+installed programs) ) over to the new box. Boot managers just complicate things and are prone to errors.