You should just use the lowest QPI multi when o/cing mate, on the Gigabyte UD5 it is 36x, however, I think it is different on other brands of X58 mobos.
With my Bclk set to 191 and the QPI multi @ 36x, it gives me a QPI frequency of 3437MHz.
I don't think there is any point in setting the multi higher as it will more than likely cause your o/c to be unstable.