After much thought, I decided to pick up the newly released and passively cooled Gigabyte GT240 1GB GDDR5 and it seems to be working alright. I can't say that it generates much heat either.
I've had one or maybe two freezes on my system where the mouse pointer still moves but has visual artifacts. Both have occurred app. 5 minutes after wakeup from sleep. I also think I have invoked the wakeup with the mouse which leads me to think that this problem not necessarily has to do with the specific graphics card or the video driver. It could perhaps be some kind of USB invoked wakeup conflict as well.
Noise-wise, I am currently focussing on the PSU of the Sugo SG05b box. I think it is a little bit too noisy and need to do something about it.