Assuming you have a motherboard that can pull ~215, and hoping for a "best case scenario", I'm of the opinion 5660 and up are the way to go. However, sku's above 5660 will bring you almost certainly nothing that the 5660 won't. We've seen that the absolute wall for these things is just shy of 5GHz (also assuming we aren't talking about a pure 1.75+v suicide run and sub ambient cooling). A 5660 @ 23*215 will put you right at 4.9GHz. 5660, 70 and 75 are all a crap shoot in the cpu lottery IMO, if you must feel like you have to have a shot at 5GHz the 5670 and 75 are probably a better bet as they will give you a little more BCLK leeway. It's still going to be a lottery though, as we've seen Burpo might have the best of the 1366 Xeons on Anandtech, cooling and the lower multiplier of the 5650 seem to hold him back. My 5670 was better than both of the 5675's I had. So much for binning.