The power connectors are pretty much on top of the cards these days and to my knowledge on pretty much all 6870's so that shouldn't be an issue, but yeah it will be a tight fit regardless.
Go with either 2x4G or 2x2G memory, since using 2 sticks will double your memory bandwidth because of the dual channel memory controller.
Also 1333MHz will be sufficient for speed for the memory as that is the maximum of the H61 platform.
As for if it will be able to run GW2 at 1080p maxed out, quite hard to say. I'm sure it will run, but you might not be able to have all the possible eye candy on. Also i doubt you will be running at 60fps+ if there is a crowd around you.