First, it's compatible with your board, but for 1366x768 it's way overkill, and you cannot crossfire any card with that board.
A 7850 is more than enough for that resolution, only if you planning to upgrade also the monitor it's valid to buy that card, it's a good one.
Browse here for uk sites with 7850 : http://pcpartpicker.com/uk/parts/video-card/#c=81&sort=...
550w is plenty for a 7870, even for a 7950, the xfx is a quality psu, no need to worry.
If you buy that 1080p monitor, you'll game in that one, the other could be for general browsing.
So if you're buying that monitor, the 7870 LE is a valid and good option, it's basically a 7950.