anandtech tested that phenominon a few weeks/months ago, I think it was in their review of the nf3 250gb reference board and they found that there is a performance boost with the nf3 and the gffx 5950, I dunno if it is the same for the 6800
From what I understand there is a performance boost, but it's not noticeable, like the difference between 400 and 410fps. nVidia has decided to add coding to make both the nForce and GeForce chips run better together, like having 2 feet, instead of 1. You can still move by hopping, but if you can walk, it'd be better, with not as much effort. Or kinda like putting a Ferrari engine in a Porsche.