PCIe has a greater bandwidth than AGP but it does not make a difference because current video cards still do not use up all of the bandwith of the AGP slot.
another advantage of nForce4 is the capability to use SLI. but due to the fact that SFFs cannot even fit one large vid card (6800 ultra), there will not be enough room to fit two video cards for SLIing.
so, there will be no advantage as of yet. unless they change the size of current SFFs and make them large enough to hold two video cards.