I wonder about the same thing, I have a laptop that's all pulled apart, upgrading everything in it(CPU, ram, SSD, BD Drive, and possibly the video). I have a 970m in it atm. Problem is, based on the specs, a 980m isn't even as potent as a stock 970 desktop. I'm glad the new Nvidia graphics are on little replaceable cards. They appear to be the same between different makes, so I think they're universal.
I'd recommend you do what I'm doing, buy a system that's got upgrade potential, that way you get more for your money, when you sell and upgrade stuff. For instance, the cpu. System I bought has 4210m chip. It's presently selling for roughly 175 bucks on ebay. They chip I want is the 4940mx. It's selling for 485 and up on ebay. To have my system built, originally with the 4940 would have cost me like 800 bucks more from Cyberpower. See where I'm going here? If you're comfy digging into the bowels of a laptop, you can get a lot more for the money. You'll void the warranty, but wth.