lammert - 6:11 am on Aug 17, 2010 (gmt 0)
CPU usage and load average are two different items. The load average is the average number of processes which are in a state that they could run, but they don't necessarily use the CPU at that moment. Processes which are waiting for disk I/O are counted in the load average for example but they don't use the CPU.
On systems with a lot of I/O you can therefore have a high load average with an almost idle CPU. If the CPU usage stays far below 100% continuously there is no reason to upgrade the CPU. But if the load average stays often above about three times the number of CPU cores, you may want to optimize your system. In many circumstances optimizing the I/O hardware (adding a RAID controller with battery backed up write cache for example) will have more effect on the load average than adding more CPU power.