That motherboard isn't really designed for Crossfire, it would run at x16/x4. I would get the Extreme3, that runs at x8/x8 so it won't cripple the second card. For Crossfire, you will want 600W+ so 750W is fine I guess.
EDIT: Personally I wouldn't game on a wireless connection, they can be slow and unreliable. If you have to, I would stick to a reliable name like Cisco.
I would try www.aria.co.uk too, it tends to be the cheapest for me.