I usually get 900+ upon embark on a 4x4. I have it capped to 200. Incidentally, if you set all your digging designations right after embark at 900fps your miners will dig out half your fort in about a minute.
Currently with 180 dwarves and a multiple biome river I still get 200 (capped), with temp etc. turned on.
i7 920 @ 3.4GHZ, 6GB 1600 OCZ DDR3. Barely uses more than 20% of my cores though. Wish it threaded better.
DF doesn't "thread" at all. It's single threaded, and a long-discussed thing about whether Toady should work on multi-threading (I wont go into that here, there's several threads here about it already).
Open up task manager, and set the affinity of the DF process to one "core" (I7's are hyper-threaded, which creates virtual cores for each real core - the game doesnt care whether it's real or virtual though). If you set the game to 1 core, you'll see it eat 100% of the core. Set it to two, 50%ish, etc, etc. I say 50ish because there is some overhead associated with tossing the execution of that single thread across cores, as well as said core being used by other things like the OS.
The OS task-swaps that thread across all your cores for a couple reasons, generally overall load-balancing, which in turn also helps reduce the strain on the actual physical core (basically, running 1 core at 100% is going to make that one core run hot, which is innefficient, and potentially damaging to the core itself). The (usually) minor overhead of swapping tasks around is worthwhile when compared to the power usage and inheritted heat (and risk of cooking the hardware).
DF is "CPU bound" - which is to say, Core speed is king, as long as you dont have any excessively slow other components in the mix.