i do a decent amount with fcp 5 on my 12" pb 1.33 ghz 1.25 gb ram, and it does fine.  granted, I normally plan on having something else to do while I edit, cuz the renders take awhile, and the final render I always do before going to bed or class or something, that takes forever.  So I'm sure the MDD will be good for it, it has better specs than mine on all acounts.
15" MBP late 2008