There's a slight difference...mostly in warranty, between brands. Asus, MSI, Gigabyte, and XFX tend to be highly regarded with video cards--just like they are everywhere else. (MSI's a tier below on mobos though still good).
You know what, I think you should save some cash by reusing an old case for now (I'm sure you know somebody with a case). Either that, or find a cheaper case. Are there any other websites you can buy from? What I'm getting at is...the i5-2500K is too good for just a Radeon 6870. You should either save up some money before you buy and get a GTX 560Ti or 6950 or save your money right now and buy a Phenom II x2 955BE.
Since you want to buy this at the end of October, I suggest you wait until Mid-November when Black Friday sales come out and build a much stronger gaming build for $800.
I can't really recommend spending $800 to pair an i5-2500K with an 6870. I'd sooner pair a Phenom II x4 955BE with a GTX 560Ti--because that would be a better gaming machine.