Yes; it's fine to use a fractional resolution while editing. This speeds up previewing considerably (especially when working with HD) and when working on a smaller screen there will be little or no quality difference. The quality of the final product will be the same.
Just an FYI: It's never good to edit video off of a USB drive. The FW 800 drive will be much better, but for optimal performance use eSATA or an internal drive (other than the OS drive).
When I get my new Lacie HD and edit w/ FW800, will I notice a difference?
Between that and the old drive? Possibly. Depends on your media. Despite common wisdom of not using externals, there are plenty of folks who get by doing so.
But the reduced res should not affect HD performance at all.