At 140 dwarves and every FPS-improvement trick I know (admittedly not as many as I'd like), I'm chugging along at 40-50fps. Playable, sure, but that drops to 25 on sieges, which is less so.
I have an intel i5 (non-hyperthreaded, so no fake cores), and 6GB of ram. I think that should be about as ideal as you can get for DF without specialized equipment.
Though the single core thing hurts, even non-hyperthreaded. It's arguable that the game runs better on the aforementioned Really Old PC.