Good question. Has barefeats, arstechnica or macworld done any benchtesting? Gizmodo makes reference to an article by Ramjet that says the macbook 'screams' with 6 gigs of ram, but I haven't seen any actual benchtesting.
Good God, the 4 gig stick is $640 at ramjet; the screams may be coming from people wanting to buy the frakkin' thing. If Snow Leopard let's us go to eight gigs, that would be cool. If the price comes down in 6 months or a year, that would be cooler. If the increase would make a noticeable difference in real world computing, better yet.
But for now, I'm going to wait.