Cooling capacity and electricity cost (and perhaps carbon targets) are major constraints for HPC. Here in the UK, electricity can be 50% of capital cost of an HPC system over a 4 year life. Worthwhile savings in electricity can be made by specifying low power processors (for example AMD HE processors) and other parts. Theoretical performance is clearly reduced due to lower clock speeds, but because memory speed remains the same, real world performance is less affected.

Bangs per unit (kWh) needs to become as important to HPC as mpg is for cars.

Whilst Intel will probably grumble about a quad-core Xeon being put up against a hex-core Opteron, the fact that the AMD option came in cheaper with more memory is what will grab the attention of the commercially-minded. Intel had better get those bulk discounts ready!