I have a Vista laptop and use movie maker for quick edits etc. Mine has integrated Intel 945 graphics and it works fine. A dedicated graphics card won't make much difference as its main use is to accelerate 3D for games or dependant upon model to accelerate HD movie playback.

I would save the cash and forget the 8400 Geforce Go, using the savings to get the fastest Core 2 Duo processor laptop within your budget. When you come to output your edit, its all about processor power.