Yeah, just upgrade the memory to the maximum that's allowed by one slot, to obtain big amount of memory.
You also don't want to be bothered by attached 'internal' hardware to be outside a casing, right. I guess it will be busy to protect this outside components.
Possibly max. Considering you have only 2 slots, it is almost always that the memory's max amount is 8 GB. You may buy one stick 4 GB first, then save the other slot for expansion another time. Or if you don't plan to go 8 GB, buy 2 x 2 GB kit. Btw, nowadays 4 GB is still more than enough, even for a gamer.