Thanks for the input guys. I can't believe I missed the DDR2/DDR3 thing... Fortunately I was able to correct it since the order hasn't shipped.  When planning this out, I was pretty much going for max khps, and I hadn't even really thought of doing a 6 GPU setup. I'd like to mine long-term, but if that doesn't work out as expected... my gaming rig has been overdue for an upgrade, so I figure it's a win-win situation.