Well you really can't go above 100% Hehe, it all depends what your doing. And I think the big thing is how long it's at 100%. There's nothing wrong with it spiking every now and then, but if it stays at 100% and things begin to lag, you have problems... Eh, unless of course you're doing some very strenuous process.
As the other one said. If the cpu hits the odd spike it's ok but you should upgrade if you find that what you do on a regular basis keeps hitting 100% CPU usage. Also check to make sure your ram isn't being constantly used up.