The 5800 series cards will overheat if they are right next to each other and you run something like furmark
. Other than running furmark while having poor case ventilation though they seem to operate well enough. I got my 5870 with the inverted cooler so I wouldn't have to worry about it if I upgraded latter on to a motherboard that placed the PCI-E slots closer, assuming I have two 5870s by then anyway ^_^.
If you are worried about the heat I am not aware of any inverted coolers for the 5850. You could slap a water block on them, but with your case you should be o.k just they way they are.