1) I beleive a non-soundcard firewire port will work just fine for recieving input.
2) It depends. Is your computer solely for recording/mixing, or do other things such as games with surround, etc. matter to you? I'm not sure the stance on using monitor speakers for the everyday uses of a computer in terms of wear-and-tear (wouldn't think it'd hurt). Some better-than-average computer speakers (like Klipsch) wouldn't be bad as a hi-fi second opinion for your mixes.