Is there no way to upgrade the powersupply in a MacPro such that one could power (2) 5870s?
Well you could route 2x6 pin power connectors directly from the psu, of course if you know the pin ups. A couple of weeks ago I offered the service manuals for the mac pros ranging from 2006 to 2009, but according to Rominator's own words, I could basically stick them in my arss. So basically I guess that you're by yourself now.