eBay would be the way to go for you GPU's. Upgradeyourlaptop has a good range of cards. I got my 780Ms from them and they will help you out with info on how to get the new cards to work in your system. Any computer store will be able to sell you some new sodimm ram.
It could be easier just to sell your machine and buy a good secondhand one with the specs you want. New RAM and dual 780Ms will set you back close to $2000! You'll get a better deal on 7970Ms or 680Ms though.