AMD Athlon II x2 3GHz
4GB RAM
1TB hard disk. Samsung are good, quiet and cheap.
Maybe 2TB if you are really storing a lot of audio. (do the sums...)
Consider how you're going to back it up.

That's about the sweet spot today, where more performance costs disproportionately more money, but you could spend a lot more if you can afford it. If multitasking is still slow, more memory is likely to bring the best further improvement. Maybe choose a motherboard with 4 instead of 2 memory slots, then you can upgrade later without wasting what you already have.