oh no.. overheating

[kerv]

89 240sx 98k miles KA

Here is the story:  I was driving home today about a 45minute trip going 70mph.  I had the AC on and the radio turned way up.  Passing cars as I go... then I glance at my Temp guage.  It is almost at the top!  I immediately turn off the AC and go into the slow lane and then I notice the guage is going down slowly.  So I thought, well I only have 5 minutes till my house... I can make it.  Glancing again at the temp guage and it is back to normal midway temperature.  I was thinking this was good... I just opened the windows and got the rest of the way down.

Once home, I open my hood to let it cool down somemore and my coolant lvl is way past MAX!  In fact it is almost over flowing.  I can hear it boiling or some type of bubbling noise and the line that leads to the tank is piping hot.

Can anyone tell me if this is bad, what the problem could be, or what I should look out for when driving some more?  I don't normally have the AC on... but sometimes I do.  How could this relate to overheating?

Thanks for your help
- Kevin