I'm hosting two gameservers for my friends, and I have two gameserver that I run in conjunction with my website, as well. My server has 512MB of RAM, and up until now, I've had no problems. My predicament is that my friends' gameservers tend to use alot of RAM. My host will gladly upgrade my RAM... for a $158 setup and purchasing fee and an extra $10/month. I need to find a way to limit their RAM usage to a level that doesn't clog up the rest of my server.

System Info/Control/Security software:
CentOS 3.4 (Just updated the kernel a week ago)
DirectAdmin control panel

Does anyone have any suggestions?