It is very unclear. Performance per watt on server workloads in currently shipping ARM hardware is very poor, and IO performance is terrible. Waiting to get hold of some of the new server designed systems (Calxeda etc) once they actually ship. Most ARM hardware is two process generations behind Intel. I don't think 64 bit helps that much. A large memory system will consume much of its power in the RAM, so the difference between ARM and Intel per Gig is smaller, especially as no one is likely to stick 1TB in an ARM system so you are likely to get more CPUs anyway.

It is possible it will work out, but theory and reality are so far off now that I am keeping my options open, was going to start a business in this space but very much keeping the options open at this stage.