I just successfully upgraded my GT780DXR to a 970m 3GB, I scored the card off ebay from a buyer who had removed it from an MSI GT72. I've had success with a 770m as well before using the 970m. I would be really cautious about buying one of the multi-manufacturer cards as the MSI machines are very picky. I had a Dell branded 770m the first time and it did not work at all, ended up having to use an MSI branded card. You can flash the vBIOS of the Dell and Clevo cards to get them working but it's a painful process as you probably know.
Also as far as I am aware the Clevo-specific cards will work in an MSI, but you will need modded INF files every time you update your drivers.