Desktop motherboards are designed with varied hardware in mind. Further, most video cards these days use external power connectors that go directly to your power supply. Laptop video cards don't have this option so have to pull all thier power through the motherboard. The 7800gtx requires more power than the 6800u, so cannot work with motherboards designed for the latter.
Clevo built the 9860/80 motherboards with what they thought would be enough to support newer video cards, but it turned out not to be. So not only do you need a new motherboard, but a new powerbrick to run the 7800.