As a complete noob to the opteron series what is the difference between the 23xx series and the 83xx series that results in such a huge price chasm between the two?

If you type in Opteron and then the CPU numbers into google there are a load of places online already taking pre-orders and the prices are inline with what has been stated above. But no one has listed a decent product specification yet so I was wondering what the differences between the models are other than just clockspeed.