When I make my new build I will have a minimum of 8, preferably 16 to 32.
The reason for this being is I "love" to host servers. Games like Minecraft that can have a maximum of 500 players on a server at once need a lot of ram to hold those amounts. I usually just play with 2 other's than myself. The maximum I've hosted Using 2G of Ram allocated to my server is 4 including myself. On Minecraft Classic I can host 8 people with 1G of ram but no less. I once called all my PC Gaming friends and we all played on my Classic Server. I then proceeded to give them all gun rights and we just kept shooting each other for a long time... Still no issues. Remember you will need a good processor for a server also. Ram isn't enough to run a fast server. And Only top end games require descent amounts of ram these days. BF3 recommends you have 4GB of Ram to run smoothly so if you wanted to run a public server on top of that with 16 players I would say you need 16GB Ram to run smoothly without any lag. Also Servers will need enough processing speed to be able to do process players actions without anymore than 10ms of lag from the time the context reaches the server to the time just before it leaves back to the client.
That's covers the basic stuff to do with RAM and Next Gen Gaming/Hosting.