QPI and DMI are the bus lines the cpu uses to connect to the memory, northbridge and southbridge on the X58 and P55 chipsets, with the northbridge and southbridge combined on the P55. While they are different, QPI offers more bandwidth, DMI with the P55 must work quite simular. If you'll look at a CPU-Z screen of a i5 750 you'll see that it shows a QPI Link.
And here's a screenshot from the EVGA P55 Classified Manual that shows a QPI Frequency Selecter, a multiplier. The Base Clock x QPI Frequency Selector = The QPI Link shown in the CPU-Z screen.