Yeah, you can replace it with an ethernet card...the only other type of card ever made for the Comms Slot. Keep in mind, you'll need a CS2 card, rather than a CS1 card due to the system's PCI architecture. Removing the modem is easy, pull the motherboard out (if you haven't done it before, it only takes a couple of seconds, remove the plastic surround that covers the expansion slots and ports, remove two screws, fold down the handle and pull the board straight out as if it were a drawer), and gently pull the modem up. The port bracket may have a tab on it that prevents you from pushing the card out of the slot when connecting a phone cable...if it does, pull the card straight up from the back gently on an angle, until you free the card completely from the slot, after which its pretty easy to work out how to finish the job.
Also, if the card was installed at the factory, there will probably be a plastic cover over the modem port (since having a CS modem installed prevents you from using the modem port), so remove that if you want to use serial devices on that port.