For a 5850 I think that's right, you should still be OK at 8x/8x. It's mostly the high-end dual GPU cards you have to worry about saturating an 8x slot, though for some reason I THINK I remember reading that with the 5870 you lose something like 4% performance. So no, not really a big deal.
Honestly, though, I would skip that motherboard and just get one that has USB 3.0 and SATA 6gb built in, so you don't have to mess with controller cards or anything like that. Just an added expense and more things that can go wrong, if you ask me. Along the same lines, I'm not sure you need a RAID controller card either, or RAID at all for that matter.
Anyway, for what you're talking about, you might consider this board by Gigabyte instead:http://www.newegg.com/Product/Product.aspx?Item=N82E16813128415
It's only $15 more, and you eliminate the expense of a USB 3.0 controller, so you'll probably actually save money. Also, it operates at 16x/16x if you've got nothing in the third slot, so you can put whatever you want in crossfire and not worry.
If saving money is an issue, there's this one with only two PCIe slots (16x/8x) for $60 less...http://www.newegg.com/Product/Product.aspx?Item=N82E16813128416