what format allows me to render ONLY the changes I make? mpeg has to re-render everytime right?

DV-AVI will only render things that changed.

perfect, thanks.

It depends on the SOURCE. There are many formats that "smart render". If you start with DV-AVI, as was mentioned - rendering to DV-AVI WITH NO CHANGES (i.e. effects/crossfades/etc...) will smart render by copying the unchanged sections.

So what is your SOURCE? Or are you going to change the source based on the answer?

Smart rendering is good for checking work in progress, but I would not recommend it for the final product. I've had audio sync issues about 50% of the time when smart rendering was involved. I've never had a audio sync issue when a full render was performed.

That's my experience, anyway. YMMV.