My 7950 has an open-shroud cooler and it works just fine. 67c under gaming load, and it doesn't seem to be affecting temps on my drives or CPU either. Maybe a blower-type cooler would do the job with less noise.
The 780ACX should fit, provided that your power supply length(?) is <=160mm. Even then, with a 160mm PSU, you need to make sure that the card won't block the modular connectors. For example, I have a 10.5" card and use a 550w Seasonic G-series (160mm) in my Node; it works because the modular connectors are on the bottom edge of the PSU, allowing the cables to be routed underneath the graphics card.
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