It depends, dual processor computers can be very nice IF the software is designed to utilize them. To the best of my knowledge none of the programs mentioned above take advantage of 2 cpus.
On the other hand, 2 cpus are always nice.
In fact, I use almost all of the above software, running it on a P3 600, and have no problems at all. I also beat my machine up pretty well, putting it under a lot of stress with 10 to 15 tracks, even if it’s a simple tune, so you should be fine.
If you use any external instruments you record on your computer (guitar, bass, drum machine, keyboard), then I would advise to spend your money on a good soundcard with 1/4 inch inputs and a small mixing board, instead of a 2 cpu comp. You will get far more productivity for your dollar.
I’d say no. Dual processors are great when you have two or more CPU-intensive tasks running at once, but music production is more of a single-task job. Take the money you would’ve spent on an extra CPU, and use it to buy more RAM and faster hard drives.
I’ve recently been talking with a friend who is interested in computer composing. From what I’ve read in trying to help him, smaller, fast seek time hard drives are usually recommended, but I haven’t seen anything about dual processors.