Under OSX a DP 867 should be noticeably faster then a single 1.0Ghz CPU. The 1Ghz FW800 was the one with 133 FSB with the later models being 166. I upgraded my MDD DP 867 FW400 to a dual 1Ghz (also swapped the board for 166 FSB).

If you swap boards you can also run the Xserve G4 1.25/1.33/1.4 CPUs. I have a 1.33ghz single G4 Xserve CPU that was sold as a DP by mistake, but kept the DP 1.0ghz.