I went back into the BIOS and turned off ACPI 2.0. I tried to disable ACPI APIC as well, but Windows wouldn't boot without it.
I re-installed, and set up Scope 4.0 (I had been using 4.5 with 4.0 keys).
Now I've got 14 stereo instances of Masterverb loaded in, which is where I ran out of DSP

I'm glad I checked! I'm also going to get the P4PE running too, as I found that turning certain things off in the BIOS would prevent it from booting (you know, the typical things you disable when you're in "computer refinement mode", such as serial and parallel ports). I think that's at a later date though, as I've now got a system I can play with! 8 cores will be useful (XP sees them), and I may try and gain access to the remainder of the RAM in the system, but again that's for another time
