All my cameras are SD DV.
My latest project includes lots of reduced size SD clips so I tht I should edit in 720. Fuck me it is soooo slow on my core duo.
You can almost forget anything more than basic cutting - starting to use colour correction and other fx makes everything sooo slow.
Personally I would say if you want to edit in 720 you MUST have 4 fast cores - the latest kit b marks up to 5 times faster then what I have - that might make it bearable.