NForce 4 Ultra is virtually the same as NForce 4 SLI, and even though they both have 2 x PCI-E slots, the Ultra cannot SLI (without being modded). As far as I know, a connection is severed on the chipset to disable SLI, and thats the only difference. You can reconnect the connection to enable SLI, but do it at your own risk.
As for the best overclocking motherboard, Probably the DFis. Although I have to say that the MSI Diamond is showing excellent promise, out-doing DFI.