How much ram would I need for a minecraft server for two players to play Hexxit without lag?

Ram isn't your issue.
ram is just physical memory for your tasks. 4gb of ram would suffice for most games and be more than enough for a minecraft server.
8gb of ram is what youd need to run All games. And multiprocess.

what are your computer specs?

for a decent mincecraft server. You need at least a dual core. Running at 3.0ghz
and a solid internet connection. Of at least 20mbps.
also remember ALWAYS run wired when running a server or really dealing with games in general.
and you should be golden