Ways to increase VPS performance?

I have a vps running centos with 4GB ram, a dual-core processor @ 4GHz, and 150GB HDD, and I've started to notice that the server's performance has decreased significantly. It starts to lag when just a few people are on. I'm pretty sure it has to do with the fact that my map is 3.1GB . So at this point I'm looking for any methods that could help boost the performance of my server. From my understanding you can add more arguments to the start up script that would help boost performance, but I don't want to take any risks experimenting which is why I'm asking here. Currently my start up script is

Code:

java -Xms1024M -Xmx3328M -XX:+UseParallelGC -jar craftbukkit.jar nogui

Is there anything else I can add to this to help boost performance or anything else I could do to help, like ram disks and etc? Please help.

I'm also running sun java 1.7.

EDIT by Moderator: merged posts, please use the edit button instead of double posting.

 

What plugins do you have? You should also make a new map for 1.3, I'm willing to bet some of that stuff is abandoned. Just an idea.