You need to provide your system specs if you want a good answer
It will fit any board with a pcie x16 slot but you would want it to be pcie 2.0 or higher. Card height is standard, you'd have to measure to make sure it would fit in your case but it will fit in most micro ATX or higher cases. You need to make sure your PSU is a good brand and can supply the necessary power.
You can only crossfire a 7870 with another 7870 or a 7850... But you'd only WANT to do it with another 7870 or you would hurt performance.