I'm not very familiar with upgrading laptop GPUs, but I would think that the power requirements would be a serious issue. If your laptop power supply cannot handle the 680M, then it won't work. Looking up the 680M, it seem to have a TDP of 100 W, which seems high for a laptop. It looks like the desktop version of the 4850 has a TDP of 110 W, and I'd imagine that the mobile version has a significantly lower TDP.
I'd recommend trying to find out how much power draw your laptop can handle, or at least finding out how much power the mobile 4850 draws. If the difference is more than 10 W, then the 680 will likely not work, since laptop manufacturers do not consider upgrading the GPU when designing the system.
Edit: According to http://www.notebookcheck.net/AMD-ATI-Mobility-Radeon-HD-4850.13975.0.html
the 4850M has a TDP of 45-65 W. I would guess that the laptop manufacturer might design up to 85 W, but I really not sure a 680M is going to be powered by your laptop. Sorry man. I suppose it's conceivable that you could find a way to downclock the 680M to be within the power limits, but I have no idea how without the GPU running in the first place.Edited by moldyviolinist - 7/24/13 at 12:30pm