Me and my little brother ran a home server on my old desktop for FTB infinity for awhile. I assigned 5gb since that was the max (due to other running programs etc) it really comes down to how many people are playing on it. 6gb would likely be fine for a full 20 person server IF you have enough CPU power on hand. I do not recommend running the server on a computer also running the client for a player. It will cause lag for everyone involved and over stress network capabilities