The 6870 costs $160 new, so why buy it used at the same price?
You can get windows 7 for $65 if you know someone who is a student with the student discount through microsoft.
The antec 302 is the newer version of the 300 and has usb 3.0 ports on the front among other improvements but is $70. http://www.newegg.com/Product/Product.aspx?Item=N82E16811129180
Even better, I got windows 7 Professional for free from my college via the MSDN Academic Alliance through my school, and its just a piddly community college, so chances are if you're a student, or know someone who is, its available to you.