Part swap on Alienware 18

Hi! I own an Alienware 18 with the following parts: 2x GTX 780M, 24GB DDR3 RAM, Core i7-4700MQ 2.4GHz, 750GB HDD + 64GB cache SSD. I know I can swap out RAM (I'm going to target something like 32-48GB later on) and the hard drives (will get a bigger SSD and make it my main). The question here is - can I swap the GPUs for 880M's and swap the CPU for something better? I'm positive the answer will be yes, because it's how you people put them together in the first place, but where could I find the cards and where do I go to get them replaced (although I can also do it myself, have dismantled many laptops in my life). How much would two 880M's would set me back? The reason being is because I do quite a bit of graphics work on this laptop and I could really use those 8GB cards for GPU pathtracing rendering (think 3Ds MAX and iRay). THANKS! (and a side note - the alienware website boasts 880M's having 16GB of memory, but they only have 8. Doesn't matter if it's two cards - still 8 usable GB).