Get a second hard drive for Vista and use the old drive for storage.
This is the safest way to go.
Anyway, on what would you do your backup? (CD, DVD, USB KEY, ...) If you have gigs of data, could be complicated and take a while.
You can also use the built-in "Accessories - System Tools - Windows Easy Transfer" to move on your new HD all your documents
on the old HD.
* Are you getting the exact same Mobo? If yes, your current HD would still work fine (but may require a reactivation).
* Is it XP what you had?