> I am planing on putting together a rack unit to use as an effects
> For a sound card I was thinking of
It really depends on the kind of instrument/microphone output and
mixer input you intend to use. You might want balanced XLR inputs and
outputs in a rack unit, mic preamps, or 1/4" jack plugs for an
instrument like a guitar. In that case you could use something like
an M-Audio Delta 1010LT. A DJ could use unbalanced RCA (aka phono
plugs) so an M-Audio Audiophile PCI would do the job.
I figure if it's a rack unit for live use then you don't want to have
any USB or other dongles hanging off.
> As this machine is going to spend most of it's time doing DSP, I
Depends on how many plugins you need to run at once. I can run Jack
Rack on my PII 300MHz laptop with 160MB RAM with a couple of simple
plugins. Your best bet is probably to prototype your selection of
software on a desktop machine before building your rack unit - then
you'll have an idea of the resources you'll need.