If you downclock the memory significantly, you will notice a drop in temps. But the main reason they have high temps in the first place is because they can't breathe in one mobo. I have two extra fans blowing at my 5850 and 5870, cause the 5850 does indeed go over 90C
I agree with dikidera, I have 10x5850's and I use floor fans to cool them down while they are hanging from PCI-E extenders. The hottest I let them get is 85 C. But majority are in the low 70's C.
http://www.target.com/Lasko-Speed-Box-Fan-20/dp/B0014IGKBU/ref=sc_qi_detailbutton