I believe the feature you're talking about is included with Logic Pro 7 - only, and you can only use Logic's built-in plugins, not 3rd party AUs. They call it "Distributed Audio Processing". However, you could run something like an Altiverb on it. You'd have to verify that Altiverb can run stand-alone, or get a really cheap hosting program. Also, you'd need an audio card with digital i/o (but analog would also work) and then you could use it like an insert on the other Mac using PT. It would be more complicated with multiple plugins running on it as you'd have to use an audio card with multiple i/o and assign each plugin to a different i/o port. At least that's the only way I know. Maybe someone else knows a simpler method.