I have a 2.5Ghz G5/1GB RAM/OS X 10.3.9

I've been looking at a lot of online forums and I haven't been able to find an exact answer to a couple of questions about my hobby studio that i want to set up. I'm looking to get a M-Audio Keystation Pro midi controller along with Logic Express and Reason 3. I originally was thinking about getting one of the audiophile cards (2496 or 192), but I find its lack of a stereo headphone out disturbing. I don't want to spend too much on a soundcard right now because I want to keep my options open down the road with something like the yamaha ox1. I looking at just doing soft synths for now, then down the road have something that will allow me to record vocals and guitar. So here are some questions that I have collected in my searchings:

1. What kind of latency numbers translate into a noticeable difference in the real world and what kind of latency can I expect with this setup and the on-board sound card on the G5.

2. What other kind of "low-cost" solutions are available that allow for headphone monitoring. I see some possibilities with the firewire interfaces, but I don't understand how they help with the soft synth processing in order to reduce latency. Do they do it in the same way a PCI based soundcard would? Do USB interfaces also do that or does an interface just allow the connections and leave all the processing to the CPU?

3. Speaking of firewire interfaces, does something like the ox1 help with the sound processing also? Does the latency change if I use the USB connection on the keystation vs plugging it into the ox1 with the midi cable?

While I'm at it, I might as well ask about about the monitors I was looking at, the Roland DM 2100s. I was wondering about the option for SPDIF inputs. Is this something useful? I don't see it on other monitors/speakers. How would it compare to the TRS inputs?

Anyway that's plenty of questions for now. Thanks in advance for the help!