I have a feeling this is the kind of thing that's just dependent on the video card. On most modern cards, a flip should be as was said above, just changing a few registers to point to a different location in vid-mem to display on the screen. That would be the fastest thing it could do.
On an older card, really low end card, it might not support actual page flipping, in which case the DDraw drivers are going to make it look like it does and do a memory blt. This'll be slow, but I really don't know if there are any cards this crappy still in circulation
So basically, just use the primary/backbuffer idea. Draw to the backbuffer, and flip it over to primary. The only downside to this is feedback effects and stuff like that, they aren't really feasible when doing stuff like this because you end up working with old data every frame
Jonathan