New computer/ new backup?

[dazzer]

I've got a new iMac arriving next week which will have Snow Leopard (bought now to avoid having Lion pre-installed as I've got a lot of 'mission critical' software that might turn out to be incompatible). Currently on a MacBook, also with Snow Leopard and I back up using an external FW drive and SuperDuper.

When the new iMac arrives I'll get my stuff onto it via a FW connection to my MB using Migration Assistant. Once I've migrated all the stuff I want to (which will be most but not all of the stuff on my MB/external backup) how do I go about using my external HD/ SuperDuper to start my new backup regime with the iMac? Is it a brand new backup or is it simply a smart update?

Also, is there anything else I should be aware of?

Thanks for any help.