Well keep in mind that just because two cores are both 3.2 Ghz doesn't mean that they're exactly the same speeds. Also understand that for the past several years, processors have hit a 'cap' at about 3.0 or so, and can't go much higher without overclocking. Basically, barring a revolutionary transistor technology (memristers?) or a huge advance in quantum computing, DF isn't going to get absurdly faster any time soon. Processors haven't gotten -that- much better over the years. Yes, your old pentium 4 is probably better, and I really don't think it healthy to spend thousands of dollars on a new system just to play DF.. Unless you're just crazy rich. Definitely build your own system, though, and don't buy some crap at a store with massive markups. Besides, if all you're concerned with is DF, you don't need a crazy video card.. Though again.. thousands of dollars for DF..erm..
Turning off temperature will get you a high boost in speed. And I mean really high.
Also, the amount of crap laying about your fortress matters too. I really don't understand these people who 'buy out' caravans. Like that's a good thing. Keep your fortress items under control. Thousands of stone crafts compared to one high quality serrated disc to sell -affects your framerate-. Use up your stone, and get rid of what you don't need via atomsmasher (drawbridge).
From what I've found, the amount of dwarves in my fortress has significantly less to do with my FPS than the amount of crap I have and temperature.