The new intel i7 cpus have a new memory interface, QPI, or Quick Path Interconnect (edited, had the wrong acronym before), and the ram interface is much quicker, and if you are going to be running simulation software, the ability for the CPU to pass data back and forth the ram is very important.
I would not suggest getting a dual core, as you are opting into a legacy socket that has reached the end of it's life cycle. The i7 CPU's have a different socket, but it appears to be a short lived socket as well, as the 6 core CPUs are going to a different number of pins. AMD tends to stick with the same socket for a longer period of time.
I did the opposite of what Tommy suggested, and put all my money into the CPU, motherboard, and RAM, and cheaped out on the GPU. I bought a 250 gts, and it does everything I need. I get 70-100 fps in 1900x1200 on most games, most importantly iRacing.
I can't run Crysis in full detail at a decent FPS, but that has been the only hiccup. I can always get a better video card down the road, but replacing the CPU, MB, and Ram is much bigger cost. I can run it at 3/4 detail, and it's more than fine.
Every other modern game I have tried to run, runs smoother than polished glass. Mass Effect 2, Left for Dead 2, Assassins Creed 2, Modern Warfare 2, (lots of twos), rfactor, name it.
While not a lot of software is multithreaded, more and more is every day, and no new software or game worth it's salt is is being designed around a single core anymore.
Outside of gaming, more cores = more options. I can run Vmware, and run Ubuntu in there, and throw two cores at it, and keep 2 for Win7. I could keep two, and tun FreeBSD and Ubuntu at one core each, still plenty fast. Run multiple simulations at once. It's like having two machines in one, and if you are running sim software, it runs better, or only sometimes, in Linux AFAIK.
Buying lesser core hardware aside from the GPU will limit you in the future, and a major purchase should be looked that way. GPU's tend to take major steps pretty quick, get upgraded often 2 or 3 times in the life of a box.
Also look at your daily computing needs. I can unrar a 1.46 gig (standard internet xvid download) file in less than 15 seconds on my i7. My compiles take less time. My multi threaded video converter, Handbrake, converts a high def movie to iPhone format in minutes. I though there was a mistake the first time I ran a conversion. My audio creation software can run more instruments, at a far lower latency. Photoshop absolutely screams along.
You must Futureproof.
Also important to see what a CPU can be OC'd to safely. the i7-920 can go to 3.2 up from 2.6 out of the box, with no upgrades to the heatsink. that puts it in the range of the $1000 dollar bigger brother.
Figure your needs first, your budget second, and buy accordingly.
Before I do anything I ask myself “Would an idiot do that?” And if the answer is yes, I do not do that thing. - Dwight Schrute