For realtime FX processing on a Nubus machine, I would suggest picking up a Digidesign Audiomedia II. Good quality stereo ins and outs, S/PDIF digital I/O, and an onboard 56MHz DSP. They also don't tend to sell at such extreme prices as the ProTools gear.

There's a realtime modular patch software synthesiser called TurboSound (or TurboSynth), although I haven't had much luck keeping it up and running (could be an OS/app mismatch). There are also some realtime FX programs around for processing an input straight out to an output. Deck II is a multi-track audio recording and editing suite which includes realtime FX on ins and outs if you have an AMII.

Best of luck. If you have trouble finding any of the software, pester me by PM in about a month when I should have my main machine (and bookmarks) back up.