it could contribute to heat buildup which will drop OCing performance or effect the card negatively in games etc, but you can monitor temps to control this. how close will the 5850 be to the 5970?
are you thinking tri-fire? 3 cards usually doesnt scale well because your bottleneck becomes your CPU so your 3rd card is not really used much... you would need to OC your i5 a lot to even begin to take advantage of a 3rd card.
also, what powersupply do you have? i assume its the 650w that came with your RC-690? i dont think i would trust a middle-of-the-road 650w with a 5970 and a 5850. even if that CM 650w can deliver its full wattage (many CM PSUs dont, like my old one), youre going to be pushing that thing to its limits. with an OC on your CPU or GPUs, you will probably exceed the limits of that PSU.
what model specifically is your PSU, some CM units are great, and some are... well... not so great....
Edited by IntelConvert - 1/12/11 at 1:07pm