I'm about to purchase a new iMac 27" and I was wondering if the video cards were upgradeable - specifically the HD 6770M and the HD 6970M. Since they're "mobile" versions of the graphics cards I assume that they're likely to be integrated into the board, but that might not necessarily be true. I've searched around for the answer but have come up with nothing yet. Even checked to see if iFixit has posted a dissection but couldn't find that either. Thanks much.