1) the CPU?
2) the mobo?
3) a combination of both?
From what I understand there is DDR2 1000 on the market, but no system can even take advantage of that kind of speed. I understand that memory makers are future proofing, but what gives with getting it to go that fast?
It's a combination of both, based mainly on CPU architecture.
In AMD Athlon 64 based systems, the processor determines memory speed (for the most part). When you overclock the integrated memory controller and memory in an Athlon 64 system, the only other thing that gets overclocked is the hypertransport bus.
In Intel Pentium 4/Celeron systems, the chipset/motherboard determines the memory speed & front side bus.