Dell will not sell you the part directly, and you do need to ensure it's the right part (no all GFGOs are the same even though they are all on MXM some OEMS have different form factors [wouldn't be surprised about DELL]).
Best place to get them are from a redistributor (I remember one guy in the notebook forums, but forget the name, he has a list of them). Or off eBAY.
Do a google search or something and look for the MXM replacement part.
I'm not sure if the thermal or power load would be too much, but it's likely close to the GF6800.
Do at your own risk of course, tricky fun stuff.