I often see it recommended to use zone focus (switch to manual focus then set focus distance with the thumbwheel) ... because of the notoriously slow AF of the DPs especially in poor light or low-contrast scenes.
they will also AF a little quicker if you set the focus mode to "landscape" (it then doesn't AF on objects under about 3M I think)
as for your other questions Herman, they will have to wait for someone who does street photography with a DPx
Zone focusing works best with the DP1 though. The DP2's combination of the relatively large sensor and longer focal length (longer than DP1) means that you need to close down the aperture quite a bit before you get a decent focus zone and that cuts down your shutter speed.
When I do use zone focusing on the DP2 I set it at F11, manual focus at 5 metres and set auto ISO, works fine in good light anyway.