The max your motherboard will support is 16GB. Which is fine because you don't need more than that. In fact it is very arguable that you don't even need that much. However, if upgrading, check your current deployment and number of available RAM slots first. If you already have 2x 4GB modules and 4 RAM slots (this would be standard but 4x2GB in 4 slots or 2x4GB with 2 slots is also possible). You can find out this info without opening the case with various tools, I use CPU-Z but many exist. This will also help you to match your new memory with your current memory when ordering online if it turns out that you have the expected 2x4GB w/ 4 slots total.
All of that said, if I was focusing money on a potential upgrade from the listed components I would, without equivocation, focus on the GPU. Everything else you have will see you through the 2-3 years at a comfortable level of gaming. That card is your overall weak link for sure.but neither upgrading that or your memory will help with Huge maps much. Huge maps are not troublesome because they themselves are large requiring lots of memory, rather that they have more pieces on them that must be accounted for at the end of every turn. More civs to calculate tactics for, more troops to move, more fights to resolve, etc. This is what results in very long turn times towards the end of the game which is the main drawback of huge maps. The only thing that could really help here is upgrading your CPU which is the last item on that list that I would consider upgrading.
Given this is all a bit of a Catch 22 what I would recommend you do is play a game on a huge map with all the civs and see if the turn times become too much for you. If so, maybe you have something to consider but otherwise save up for that GPU man, $140 is approx half of what you need there.
