I am using an ATI 5450, passively cooled since a long time on my HTPC. 20 W TDP, still around 20k Collatz or so. However, it's the only card not allowed to crunch since there is some lag when watching football.
Today the smallest ATI using 28nm would be the 7750. And it seems low profile. 819.2 GFlops for 55 W TDP...Just little less GFlops/W than the 7970. And double precision.
http://en.wikipedia.org/wiki/Souther...#Chipset_table