What is the best low-cost, low-power CPU cruncher?

[zombie67]

Looking for best BOINC performance per $, and lower power consumption is more important than purchase cost. All things being equal, 2 tasks running at half the speed > 1 task running at full speed. IOW, multiple threads per core is good.

Go!

Edit: Since this is a CPU cruncher, no included windows is okay. Linux is fine.

[John P. Myers]

Are you looking for the best perf/$ of a full system or just the CPU?

[zombie67]

Full system, and 1gb/thread is a minimum. So that may knock out many computers-on-a-stick. No discrete GPU required.