Selecting a machine for OpenCL / GPGPU use

What is GPGPU computing?  GPGPU stands for General Purpose computing on Graphics Processor Units.  The basic idea is that we use the graphics card to do some computing.  Today most computers have 2 cores and some as many as 6.  With hyper-threading we may see what appears to be double that many processors.  However, a video card such as the NVidia 480 has