have you updated the bios on the VNF4/Ultra yet? Cause the newer bioses are great at OCing. I have mine at 283MHz right now, with no issues. I think it can probably go higher if i overvolt my CPU a bit more. I think there might be a wide variability, though, cause some reviews say it can hit over 300MHz, while others say it tops out around 230.
Other than that, though, the DFI seems to be the way to go. Most people seem to be able to hit over 300MHz pretty reliably.