Why $500? Mac Pros do not, strictly speaking, require the official stock "Mac Pro ATI Radeon &c &c" with the mounting bracket and everything. I had a HD4870 r

Why $500? Mac Pros do not, strictly speaking, require the official stock "Mac Pro ATI Radeon &c &c" with the mounting bracket and everything. I had a HD4870 running strong until a couple months ago it died. I replaced it with a GT 640 I had on hand; it works fine. Your mileage may vary.