A 3 seat pickup would loosen the 'rat chokehold on Seanate business. A 5 seat pickup along with W landslide would force the Democratic Party to reconstitute or go Green.
The democrats have several other seats that are not rock-solid locks. For instance, Blanche Lincoln (D-Ark) comes to mind. The Louisiana seat of John Breaux (D) is safe if he stays on (fairly conservative democrat - as democrats go), but if he retires, it's anybody's guess. And what about Byron Dorgan in North Dakota. Bush carried that state by over 20% in November 2000. If GW brings that margin again, there might be coattails for a qualified Republican challenger.
Although I'm still looking at a 3 or 4 seat Republican Senate gain, if Bush gets it all together with a good economy and job creation on the rise, the underpinnings of a 1964-style landslide are there.