I didn't want to read too many posts on EP3 because I've been wanting to be surprised unlike I've been with EP1 and 2 due to all the spoilers floating around. However, with the stuff I've already heard from starwars.com I believe the gap will not be 100%. Lucas wrote the six episodes with gaps between each one. There's always some unknowns that occurred between each story, like the Approaching Storm book whose plot was mentioned by Windu in the beginning of EP2.
Thus, I don't think everything will be or even should be shown between 3 and 4. We know we'll see Ani turn into Vader, the Jedi beginning to be either persecuted or hunted down, and Yoda and OB1 go into hiding. There's also that one scene Lucas already filmed for EP3 that is mentioned on disc 2 of EP2's DVD. I don't think anyone can say for sure what else is going to happen for sure unless Lucas reveals more stuff. It could be that many of the things we've speculated about already in this thread won't ever be shown on film because they'll just be alluded to.