I have been asked the question and do not know the answer. GT 520 vs 2x 8600GS which is better? Edit: Sorry guys its 2 8600Gs cards he's got.
I have a 8600gs hooked up to a bulldozer, 8g ram and a 60g ssd. I will try and play diablo on it later and let you know how it goes. edit: I am sort of just trying this for a laugh. And I also intend on upgrading the graphics card at some point.
The 520 is better, 48 cores versus 32 and higher clock speeds. Not to mention newer so the cores are more efficient/powerful.
The 520 for the same reason as above also I couldn't find the 520 (Maybe I am going blind) but here is the OEM 8600gs compared against a 420: http://www.hwcompare.com/6874/geforce-8600-gs-oem-vs-geforce-gt-420/