Dual slot cooler

Sorry if this question has been asked before but having trouble with the forums search function saying things don't exist.

Anyway I've been pondering over building a mid range system in an Antec Fusion case (yes I know not even the best HTPC case for air flow but I need a case under 16cm tall preferbly a inch or two smaller like the antec for air flow around the case), planning on swapping the case fans out and only using a core duo with a scythe mini ninja in an attempt to keep things cool. However when it come to graphics card my original attention was to use a HD3850 as that seemed to be the best one that would fit however I've read else else where that you can shoe horn in an 9" card so that should include the 8800gt or gts (please correct me if I'm wrong).
To get to the point am I right in under standing that a dual slot card will exhaust most the hot air outside of the case unlike a single card? And if so are there any 8800gt cards with a dual slot cooler or after market dual slot coolers I can use to save me forking out another £50 for the gts and only getting a few more shader pipe thrown in?

Thanks for any advice.