I would agree that the Athlon64 is over the top for your requirements. Even though you can never have too much CPU power for softsynths you would be paying way too much money for the chip / motherboard / memory as there is always a premium price charged for the latest components.

Regarding AMD or Intel - AMD usually more bangs per buck BUT there remain some software packages (usually specialist) which insist on Intel processors. I dont think this will be an issue for you but it may be best to check the small print of your preferred softsynths carefully.

Intel claim that "Hyperthreading" in their newer processors improves performance. This can be true IF the software being run has been written to take advantage of this feature; all benchmarks Ive seen so far show the reverse to be the case for un-optimized applications; i.e. the PC is a little quicker with Hyperthreading turned off!

In your shoes I would be looking for the fastest AMD processor I could afford, with 512M of memory, using a fast front-side bus.