Although modems + linux have come a long way, I am almost certain the USB external modems are winmodems which are hit or miss with linux. These and internal PCI winmodems can be setup in linux, but it is not for the faint of heart. You could find drivers tomorrow or you could still be looking two weeks from now. ( I have done both).
May I recommend an external serial port modem. Almost all of these are hardware modems and drivers are built into the kernel so no fuss-no muss. US Robotics or Best Data ($15-$100). I personally buy them at the local thrift shop for $5 or less.
Use wvdial or Kppp to setup and connect via dialup. Watch out for AOL. It is NOT linux friendly AFAIK. Linspire has an AOL script (but is only for Linspire users) and something called penngy (for AOL) has been floating around but I could never get it to work nor have I heard of anyone else having any success with it.
Haven't used dialup in a while so I don't have some of the settings off the top of my head, but don't set speeds too high. You will get fast page loading, but downloads will choke and stall the modem. 3.2-4.6 kb/s is reasonable.
hope this helps