I think I know what's needed for a decent rig that requires lots of programming (and OpenCL) power, or a high-power server. THIS IS NOT A GAMING MACHINE.
Case: A case that delivers lots of cooling power
CPU: Intel Xeon or i7 (depends on what you're going to use the rig for)
Motherboard: A multi-socket server motherboard with a high bus speed (single-socket consumer if you want to use the i7, because intel bans multi-socket consumer motherboards)
RAM: 16+ GB high-speed RAM
Graphics Card: Some nVidia card with lots of CUDA cores (preferrably not a gaming one that is actually a toaster) or none at all (depends on what you're going to use the rig for)
HDD: Some hybrid drives hooked up by RAID, or just one (again, depends on what you're going to use the rig for)
