Server Crashing - "java.lang.OutOfMemoryError: Java heap space"

Hey guys! I've been running a server for quite some time now, but just recently I've upgraded the server to Ubuntu Server Edition, But now once or twice a day the server comes to a grinding halt with this error!

I have tried assigning more and less RAM but neither help!
My system is as follows:
AMD Dual-Core Athlon II X2 3.0Ghz
8GB DDR3 RAM
500GB HDD 16mb cache

CraftBukkit 1240 (Not upgrading until 1.9 comes out)
10 or so plugins
2 Worlds, One of them loaded into a RAM disk only assigned 512MB

A Mysql database and an apache webserver is running.

When I run minecraft I set it to use a max of 4GB yet for some reason it is running out of ram! It seems like linux is using alot of RAM for cache (System Cache) could this be the culprit? and how would I go about fixing this?

Please Help!