MIPS-based motherboard configured to handle multiple MIPS CPUs. Far more important is that actually using MMX instructions requires that you be very nearly coding in assembly language in order to recognize instruction patterns that might be sped up by replacing them by an MMX instruction. As a result of that, as well as the consideration that CPUs and assemblers don't universally support the MMX instructions, the usefulness of MMX on Linux has been, thus far, rather limited. Unfortunately, attempts at portability discourage MMX use, as XFree86 these days supports many platforms, a number of compilers, and multiple architectures. Real Soon Now.
Nobody seems to have written any such libraries yet.
Unfortunately, this is highly unlikely to happen. GCC would have to recognize very particular sorts of iterative structures. Context switching cost is fortunately not a problem here as the buffers checksummed are guaranteed to be large enough.