Motherboard: make sure your processor fits also, if you want a silent pc check some reviews on power usage, the less it uses, the more silent you pc can be cooled. Same thing is true for every component, including CPU and GPU (making the absurd 1kW PSU even more overkill ), so:
The 3850 is good, too, but it's so cheap right now that it can be considered budget-class, and that's probably not what Gorth's looking for. Otoh, chances for it being cooled passively are bigger than with a 3870, let alone x2...
Oh, and if you want 8GB of RAM, the mainboard needs 4 Ram-slots.
[edit]Yeah, I'm probably stating the obvious, but sometimes the most obvious things are overlooked. No harm intended