Blender generally speaking taxes a computer in lopsided ways. Usually, the most important component for Blender is the GPU, and then the # of CPU cores if you plan to use the CPU for rendering. RAM, harddrives etc are generally speaking less relevant, depending on whet you want to do.
More CPU cores helps mainly with CPU-based rendering, and certain types of simulations. Most of the rest of Blender doesn’t take much advantage of multiple cores.
of GPU’s is probably less relevant than the exact GPU that you select. Blender/Cycles is known to work best with nVidia GPU’s such as the GTX 980, because it uses nVidia’s CUDA to accelerate Cycles. This is changing though, AMD cards might one day be as fast or faster for Cycles through the effort of splitting Cycles apart and adopting the industry standard OpenCL instead of CUDA. But for now, nVidia GPU’s are your best bet. If you plan to do much rendering with Cycles on the GPU, don’t skimp on GPU memory. Many cards come with 2 gigs of memory on it, but you’ll want something like 4 gigs to be able to render most scenes. To make Cycles really fly, multiple CUDA-GPU’s can really make it sing.
Hard-drive performance is generally speaking less relevant for Blender, compared to, say, a video editing app. 3D geometry is fairly light on space compared to video, so loading times don’t tend to suffer on most scenes. Don’t bother with RAID setups or anything.
Likewise, because 3d geometry is fairly compact, you’ll rarely see scenes that take up insane amounts of RAM, though it can happen with complex scenes with millions of hairs or grass etc. And because your GPU can only handle a smaller amount of RAM anyway, you won’t be able to properly preview those types of scenes. But it depends on how large your textures are and the amount of data your throw at Blender.
Blender runs on anything, but if you are mainly (only?) using your computer for Blender, Linux is the best OS for it. Tests have shown that Blender is able to render scenes slightly faster on Linux compared to OS X or Windows.
In conclusion, in general terms, the most important component is the GPU. It’s the GPU that makes it possible to navigate and edit more complex models smoothly, and render them quickly using Cycles GPU rendering. The rest of the components all matter, but less so.
So, an ideal custom Blender-computer might have multiple nVidia GPU’s (maybe GTX980’s ?) with lot on on-board RAM, with the rest of the components just being fairly standard and you’ll be fine.
This is all in general terms, of course it will depend on what kind of things you want to do.