When Is A Server "full"?


I have a RHEL server that is begining to show signs that it's getting tired. There are times when the load spikes and the server becomes unusable. I've added a few new drives to add more space. Backups of MySQL, email and files take quite a long time, and even those can overwhelm the server.So how do I decide when the server is full? When do I add a new server, or would adding more RAM help (2 gig currently)?I suppose this is more philosphy than a tech question, I'm not really looking for specs on hardware, but rather a general rule of thumb on when I need to start thinking about new hardware.