in answer to trygve's question, modem updater 2.0 does update modems on AGP G4's with ADC connectors.
the "official" apple support designation for the newest g4's is "gigabit," and am not sure what moniker apple support identifies pre-gigabit sawtooth machines [prolly is "agp"].
http://asu.info.apple.com/swupdates.nsf/artnum/n11859 states that this updater works with internal modems on PowerBook G3 Series, iMac, Power Macintosh G3 (Blue and White), and Power Macintosh G4 (PCI and AGP) computers."
i've updated a g4500mp [gigabit] and powerbook g3 series 233/512 [wallstreet pdq] without issue. the firmware will not update an incompatible modem at any rate, as it checks the modem type first off.
i seem to be spontaneously disconnecting with less frequency, although the g4 has gone away for a new motherboard...