You can go for the i7-920XM or 940XM but you really won't see much improvement over the i7-840QM that you already have. The XM chips are unlocked so you can overclock more than the QM variants. You cannot upgrade past the Gen 1 processors. If you had an i3 or i5 then I'd suggest to go for an i7 but you already have one of the best i7's. 8gb is the max memory and I believe it's limited to 1333Mhz in the M15x but you could try 1600Mhz. It may just downclock back to 1333. The GTX 970M is a popular GPU upgrade. The 980M is possible but can have some heat and power problems. Something like a 770M or 780M is also a pretty decent upgrade and should be a little cheaper than the 970M. Any Nvidia card that didn't originally come with the M15x will require a modified display driver to get it to work. This isn't too difficult and BlazeGhost over at efgtx.net keeps the modded .inf file updated for people who have done this upgrade with the 970M [URL]  
Would like to upgrade my M15x I7 Q840 laptop. what all can I do to up grade to better speed memory, graphics ect.? would like to also change from AMD hd 5000  5730 to NVIDIA some one please HELP, THANK YOU, JohnK
M15x, I7 Q840