There really is no need to answer this question. There are only a handful of 512x512 packs, presumably all of which have various lower-resolution alternatives. The in-game difference between 256x and 512x is so small that there is no reason to use a 512x. They are uncommon because there is such low demand for a CPU-hogging pack with details so minuscule, they can't even be seen.
Well my old desktop(before it went boom) could run 512x fine. 512512 packs are quite resource intensive though, I had 8gb ram 4 o whihc was allocated to MC and a pretty decent graphics card and cpu(i5 2500k and Sapphire 7850).
I have a $1500 rig and it runs it fine. I have 16gb of ram and dual 6950s 2gb each. An i5 2500k and above or amd phenom 2 1100t or fx series processor will be great especially if you plan to record. Hope this helps even if it is late