Just to give everyone something to look at here's a few pics of an '00 Altima.
Doesn't look like too bad of a shape, aerodynamically speaking. It just needs a little smoothing out here and there.
-Lowering
-Wheel skirts
-Grilleblock (there sure are lotsa' openings up there)
-Mirror delete (it may be the pic, but they look huge)
-Airdam or bellypan
-Boattail off of the trunk
Rideheight would be a great place to start, plus lowering springs should be easy to find for this car. That would also take care of a lot of the extra gap in the wheel-wells.
As for the boattail, I don't see that much to be gained from starting it up at the roof. The transition is already pretty good from roof to trunk. Perhaps it just needs a few VG's to help like the Mitsubishi EVO.