Macs are nice machines, but if cost and performance is an issue, than, sadly perhaps, 'Doze is the way to go. I use a lot of the same software as you, and going for the cheaper e-macs or i-books isn’t going to cut it.
I don’t know if you are familiar with Max/MSP, it’s a sort of graphical programming environment for sound synthesis. On a performance test for that program the following computers scored:
Dell Inspiron 5150 P4 3.06 GHz: 46
Desktop G4 Dual 1.4 GHz: 63
Powerbook 12" G4 867 MHz: 121
Smaller values mean faster performance.
The powerbook is about 1100$, the G4 about 1600$. The Dell starts at 900$ and is three times faster than the powerbook. That means that for the same price, the Dell notebook will be able to run about three times as many vst plug-ins. Or so the benchmark test shows, but in my experience that’s about right.
Also, since you are doing CPU-intensive DSP stuff, I’d skip the celeron machines as performance is going to be not so great.