In general
Generally speaking, any interface with PowerPC Core Audio and CoreMIDI drivers — and the inputs you need — should work. Beyond this, it really depends on how many inputs you need and how much you want to spend.
One inexpensive ($250) example to start your search: the PreSonus FireStudio Mobile has MIDI in/out, along with inputs for, say, a microphone and guitar (XLR or 1/4"), stereo line-in from your Yamaha's internal synthesizer, and a bit of room to grow (four more analog 1/4" TRS inputs and one stereo SPDIF coaxial digital input, to be precise). It's also a bus-powered FireWire device, so it's easy to set up and to move between your G5 and other machines in the future (it does include a power adapter in case you ever need to use it with a 4-pin PC laptop FireWire port). More importantly, in my (admittedly brief) experience, it's rock-solid and sounds great.
