When looking at a case, ask yourself where is it getting the air from?
The H500 has a load of mesh on the front that air can get in through.
The 678C, like the 275R, doesn't. The problem with a nice, sealed up, sheer-fronted case, is that you have a nice, sealed up, sheer-fronted case that doesn't allow for airflow to the graphics card.
The 678C doesn't seem particularly good either according to
Hardware Canucks. Vortez's review doesn't measure GPU temps, for some reason.
Kitguru say that "it's not a complete disaster" which somehow doesn't seem like a stellar result. It didn't do so well in
bit-tech's review either. Tom's Hardware's review is all over the place, posting
poor to mediocre results but then praising thermals in the conclusion as if they hadn't read their own numbers.
My bottom line would be that there's not much point buying a high end PC if the bits inside, especially the GPU, will get a bit smothered.