what type of 5850 cards are they? if they're the non-reference designs that doesn't shoot most of the heat out in the back, then the card in the first slot will have the highest temps in most setups. that's because the heat goes upwards, and the this top card will recycle the heated air from all the bottom cards.

as for the memory underclocking, just use MSI afterburner and after you configure your config file, the upper and lower memory limit should be removed. i just posted on how to do that in another thread:

http://forum.bitcoin.org/index.php?topic=30032