Well, sometimes like in ATI 4xxx series it goes like the mobile version is based on the same chip as the desktop counterpart i.e. mobility 4850 is based on the desktop 4850. In 5xxx series, that doesn't unfortunately apply as mobility 58xx is based on desktop 57xx called "Juniper". The best 5xxx chip called "Cypress" is not used in any mobility cards as of now. That applies to almost whole series, 5650 and 57xx are both based on the desktop 56xx chip called "Redwood".
That's why most people are so confused. They think 5850 would be huge upgrade as it is in desktop models but in mobility cards, the difference is fairly small
if they can put a 5850 in there.. desktop edition.. i would sell my imac and get an 27 inch imac. btw if apple was to upsize the imac.. how big would the screen be due to the 16:9 ratio pixel count etc.