I would caution anyone against using a dual core processor with Windows at this time. There are strange, horrible bugs that are effecting many systems. Microsoft, Intel and AMD don't seem to have a clear idea of what is going on.
I have spent over a year trying to get just one computer -- any computer -- with a dual core to run Windows XP and Sonar. I have tried Pentium D and Core 2 Duo. The former crashed unpredictably, taking down Windows, turning off the power, corrupting files and damaging hard disks. The latter will be going along fine and then suddenly start hogging all CPU resources for both cores. Opening a file in Sonar that is normally instantaneous can take as long as 10 minutes -- and when it finally opens the CPU usage gague will be at 100% and everything except the close application button will be locked up. Even worse, sometimes Windows starts in this mode. Every service and device driver claims 100% of both cores as it loads, in turn. Earlier this week, I spent the entire day just trying to open Windows.
I have been using, building or programing pcs in some capacity for over 20 years now and I haven't seen anything this bad since the days before Windows and Macs. The worst is that nobody -- not Intel, not Microsoft, not anyone -- at this point seems to have a clear idea of what is going on.
I can't speak for Macs. I've only got one of them and I don't use it for music. But, if you are in the Windows world (as much of the world is) right now the best advice I can give is save your money until things are more stable (or if you are despirate buy the cheapest, fastest single core processor you can and sit tight until something better comes along.)