i5 2500k here. Admittedly, Dwarf Fortress was not foremost on my mind when I shelled out $800 for a new gaming computer (Skyrim and Lord of the Rings Online were, but finals are next week and I'm not risking that big of a GPA drop). Because my college's network is terrible, I haven't done much. I just sneakernetted DF on it to see what my framerate is.
I started up my 55-dwarf fort and was getting a full 100 framerate. I checked my init file, and it was capped, so I uncapped it and started again. This time I did a vanilla embark, 4x4, no flowing water, all settings pure vanilla (temperature on, 7 dorfs, etc).
I'm getting 500-550 fps. A day passes in a few seconds. I don't know if I'll limit it or not - I remember 200fps back in 40d days (the only version that my poor laptop could squeeze 200 fps out of) was a comfortable limit, in that I wasn't sitting around bored waiting for stuff to happen, but I could tell what was happening. I think that the i# series may have ushered in a new era of mega-forts. If I ever get my term paper done, I'll continue my 55-dwarf fort to see what framerate looks like on a 200-dorf fort.
Keep in mind, I'm not overclocked. The 2500k will overclock itself up to 3.7ghz, but that's automatic and on-demand (which means what if I'm running DF, it's overclocking one of the four cores). I think other contributing factors are the fact that I'm sitting on 8gb of DDR3 dual channel ram, and using a motherboard made for gaming (ASRock Fatal1ty series, although I think it's mostly bull and you won't see much difference). I was still astounded, having come from a much older setup. I haven't even optimized for framerate yet - no allocation of processes to separate cores from DF, no tweaks, all pure vanilla.
DISCLAIMER: This next bit is based on my understanding of processors, which may or may not be utterly incorrect and flawed.
At this rate, we really don't *need* multi-core support because the architectures are so good. The reason other people aren't seeing similar scales of performance is that, as I understand it, the speed of the game depends on how many calculations per <arbitrary time unit> can be made. And that depends on how many transistors there are, and how fast they can perform these calculations (which is the number of hertz, a.k.a. what they try to sell you CPUs based on). As I understand, the i5 and related processors have a whole lot of transistors, and they're a lot smaller than they used to be. Ergo, the effective speed is vastly increased, even with only a small increase in clock speed.
We really should try putting together a thread with instructions on how to build a cheap DF rig and get it stickied. In the past months, my interest in DF has been sucked away by a myriad of other games. With this speed and power in my new computer, I'm excited to get back into it. And there hasn't even been a new release!