[Zogthetroll]

regardless of overclocking or not, the gpu is still important as its one of the major heat sources, and should be cooled accordingly. stupid question, but have you tried different types of HD's? have you also tried directing a fan over the hard drives? had to ask.
about the manifold, do look at bladerunner's, all they really require is a chunk of plastic, a drill with a few appropriately sized bits, a saw, and a tap for the barbs, and maybe a file. no actual machining involved.