No, I wouldn't rely on digital. If anything, you'll be better off going for analogue monitors. They are more compatible, actually more future-proof (as digital standards are set to change) and will sound better.
If you use the digital output on your PowerMac, remember that you won't be able to use the volume control.
Minjack can sound just as good as any other connection method, just make sure you don't skimp out on a decent cable. The sound card in your PowerMac is actually of pretty high standard. Believe me, you'll be best off getting say the M-Audio BX5as (very good monitors), and connecting them directly to your PowerMac. I'm running Samson active studio monitors directly off a PowerBook as we speak, just using a Minjack > RCA cable (make sure it's well shielded though), and it sounds far more than acceptable, it sounds better than my Dell + Pro soundcard in the studio (been meaning to upgrade to a Mac out there for ages)...