Job 1: Switch out your HDD's with SSD's. (Not usually something I recommend, but minecraft is rather unique in this instance, that it does a lot of I/O.
Job 2: Check that you ACTUALLY HAVE 1Gbps/100mbps and not just a 1gbps/100mbps port.
Recommendation:
You may find it more efficient to run multiple linked servers. (e.g. Use Transporter or Bungee Cord to link multiple servers together)
Side note: Opterons are great for multi-threading, which minecraft is not. If you wanted to run 30+ servers on the same machine then this CPU would suit you very well, if you're only running 1 single server, then it is very much wasted.