Those cubes use about 220W low mode/330W overclocked from the wall. You'd think that the CX500 would be sufficient for one (and it's single 12V rail is @ 38A, which is plenty), but I keep seeing posts about people having issues with the CX series and these cubes.
I've run a single cube off of a TX650 with no issues, and two cubes at the same time off of a TX850M, again no issues. Whatever you decide to get, make sure the PSU has a single 12V rail that's rated for 50A or greater. Good to go.
I run 4 cubes on a single Corsair AX1200. Aside from melting the ends of the power cords once a week it works great, averages 155Ghash.