Over the last two years, we've been working on the X-Plane rendering engine, setting up the code to take advantage of multiple cores when possible. Most of these changes are in X-Plane 9 already: multi-core creation of 3-d scenery, multi-core loading of scenery, multi-core loading of textures.
X-Plane 10 will leverage this work, pushing even more work to multiple cores. And yet, nine women cannot have a baby in one month. The ultimate limit on framerate will be based on the performance of one core pushing data to your graphics card.
So how many cores do you need, and is it better to have a few fast cores or more slow cores?
I can't give you a firm answer, because I don't know how important money is to you, I don't know which rendering settings you care most about, and X-Plane 10 isn't finalized. (And even if it was, we often improve performance in patches.) But I can suggest our attitude to how cores are used.
A Graphic Analogy
With graphics cards, the companies target different markets. The "enthusiast" market is the top end, where money is no object and maximum performance is the goal. Below that you have "performance" cards (a good value for the money but not as fast) and "mainstream" (which by video game standards means "slower than snot" - main stream users don't need fast 3-d graphics to check email).
When it comes to rendering features, we expect X-Plane to be efficient enough to meet the performance expectations of a given slice of the market. In other words, if you have bought a top-level graphics card, we need to be efficient enough to let you run at 8x FSAA on a huge screen with per pixel lighting - that's what is expected of the enthusiast crowd and that's what the cards are built for.
But if you have a mainstream card and get 5 fps with per pixel lighting, well, too bad. You've got a cheap, low powered card, you need to dial down the sim. Here our goal is only to give you a way to run X-Plane at all. (And frankly, if X-Plane could run at 60 fps on a mainstream card, it means the max rendering settings don't take advantage of top end hardware!)
At this point we expect pretty much any modern computer to have at least two cores, and users with single core machines are going to have to make serious graphic quality sacrifices to run X-Plane. (This is already true for version 9.)
When it comes to version 10, I think we will categorize "a lot of cores" (e.g. 4, 6, or more) as enthusiast - some of the top level rendering options may not function well with less than four cores, but dual core machines should at least run decently with some rendering options enabled.
How far we go toward utilizing really high core count (Austin upgraded to the new Mac Pro and now has 8 hyper-threaded i7s) I don't yet know. My guess is that we will get at least some measurable benefit from up to 8 cores.
Trading off clock speed for core count is a difficult choice. Obviously if you had a choice of one core that was twice as fast as a 2-core CPU, you'd take the speed - you're getting the same total computing power but the clock speed can help frame-rate. But the trade-off is almost never formulated like this, and it's too soon to have hard data from the sim itself.
For what it's worth, the latest generation of CPUs is really fast, so it is unlikely that you'd upgrade to a new CPU and not get some serious benefit, whether it comes in cores or clock-speed. The users I have heard from with new iMacs seem quite happy with their machines.