if your gonna go external for Linux i would recommend a Serial Model,USB modems have a bit of work before they will be stable on linux(PLEASE SOMEBODY CORRECT ME!!!),some versons of linix require you to reboot before you can use your USB modem again and there are a few soft modems available on USB so beware!
Any well known serial modem will be fine,try to get V92 and Hayes command compatible as Linux will work better with this..or at least the versions I have used!
HTH
David