I went from 6GB to 12GB, skeptical I would see much of an improvement, and the difference blew me away.
You have to be careful just filling up extra memory slots on your mobo - many mobos optimize memory access via channels and having mismatched memory (in terms of speed) in the same channel can limit performance.
I’d suggest first figuring out which mobo you have, and then go get the users manual and memory compatibility guide for it from the manufacturer. The user manual will tell you the maximum memory capability of your motherboard and which slots go with which channel. The compatibility list will be a very conservative list of what’s compatible with your system, but gives you an idea of what memory suppliers the mobo manufacturer works with.
If your mobo is old enough, you might need to go to forums like www.overclock.net to see what experience other users are having with your board.