Depends. Do you happen to already have ADAT converters? If so, you could get an RME HDSP 9652 to have 24 channels of both ins and outs over ADAT and hook up your converters to that. M-Audio Firewire interfaces are another, cheaper option. Also the TC Electronics Konnekt 48 is an interesting option - it has a rather capable DSP chip builtin, into which you can offload effects processing.
One rabbit hole you could fall in are the TC Powercore units, which are Firewire-connected boxes full of DSP processing power so you can run plugins on them instead of your machine's CPU. They're no longer supported and the licensing infrastructure is gone, but whatever licenses were installed into one will remain usable as long as you don't disable the license yourself. I have a Firewire unit with the Access Virus synth in it, got it for next to nothing. If you do this, be sure to understand that
licenses are no longer transferrable so if someone offers to sell you a plugin license for Powercore, it just can't be done anymore. The authentication servers were shut down sometime in mid-'10s.
I have an RME Multiface on my G4 Powerbook and an M-Audio 1814 on the G5. Both work fine. The software I'm using is Logic 8.
I've tried to collect some information about audio stuff that runs on PPC and Leopard here:
Some Audio Units that work with PPC Leopard. Commercial: Valhalla DSP: https://valhalladsp.com/ These have "legacy Logic 8/9 builds" downloadable which include PPC versions, while it's not explicitly stated on the webpage. - Valhalla Room - Valhalla Shimmer - Valhalla VintageVerb Toontrack...
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