I saw the posts by Rob and James a couple of weeks ago about multicore and 64 bit but was too sick to kick in at the time. Now that the coughing has stopped, I want in. Before I left my day job to go back to try to finish that school thing, I had the joy of using a dual 3.6 Ghz processor Dell and it was a joy. While most software still is not written as multithreaded, the ability to have a processor or a core that is dedicated to ArcGIS, or any other processor hog, while leaving a processor for the OS and other low overhead software led to much less crashing and huge speed improvements. While you can set the processor affinity for any process in the Process Manager most of the time it isn’t necessary as Windows will balance processes between cores/CPUs. While this isn’t an issue if you are using ArcMap for cartography, it is great for analyses where your raster calculation, or other, gets its very own core.
Just as an example, we (ok mostly Sue) are working on a project in ArcScene that contains a high-res elevation model and a large style gallery of 3D symbols and we consistently crashed ArcScene on our single processor machines, but we bought a fairly cheap dual-core AMD and it works like a charm. ArcScene’s graphics engine still seems to have the issue that it doesn’t push enough off to the GPU leaving the CPU to pick up the slack.
The upshot is that I definitely suggest spending your money on a dualcore machine over a single processor machine if you do any kind of heavy computing, even if the dualcore is rated at a lower clock speed the process balancing between the cores will usually help you shave time off of processes. As for me, as soon as Apple releases a Core 2 Duo laptop I will be standing in line.