[Richard M]

This is why:

Keeping the VPU on the underside of the graphics board keeps it well away from the primary thermal monster in the PC - the CPU. If we put the VPU on the upper surface then its heat would typically feed directly into the CPU - and that would mean having to run CPUs slower to stop them from overheating.

http://www.3dvelocity.com/articles/a...iew/rhuddy.htm