The main heat is coming from the OC cpu's, but I'm sure the GPU's are producing a fair amount of heat as well, along with the Northbridge if Dell had to up the voltage to achieve the 3.67. The 8800's and up tend to have an exhaust port at the bottom of the card that blows down on the mobo and heats it up as well.
I'd suggest grabbing a
T.R.U.E. One of, if not the best air coolers you can get.
Not sure if they'll fit in a standard Dell case however. What cases are they in to begin with?
Like Elli said, case airflow is very important. Its one of the reasons I'm rocking this case now -
HAF 922