MySQL Performance Planning
You need to find out first what makes the load get too high so that you are able to plan ahead. 

If its IO bound then you need faster disks. If its memory bound then you need to add more memory or find ways to reduce its usage.

The best way to find out what the problem is, is to track your system during a peak time. Get a graphing application and look for:


Hard disk usage
CPU usage
Memory usage
Swap usage
Raid card, if you have one


Also keep track of your SHOW STATUS info and see what are the differences. Try to notice patterns, find the bottlenecks and predict scalability issues. 

For example, if you system is currently doing 100,000 transactions per minute and your bottlenecks are at 75% capacity, then if your TPM increase it to 140,000 your system might crash/stall/lagg/die-a-horrible-death.