I did learn something else a little while ago. I was concerned about being able to get Linux to work on my olld internal dial up modem {drivers issues etc}. I was buying a new external modem/fax modem & US Robotics makes an external USB modem that they say will work on Windows, MAC, and Linux. USRobotics USR5637 56K USB FaxModem for Windows, Mac, Linux. So when I get the time to play around with Linux I can try it out on an older computer and no harm done either way.
I used dial up for years in a apt. with 2 floors using the same phone line! For free with AOL and then with http://www.freedialup.org/site. No ads. What part of US are you in?
So i bought a used US Robotics external mod but it worked with a serial cable. I also had a device that allowed you to answer the phone without dropping your connection. When we moved last summer i gave i or threw them old away though.
I see usb robotics modem external beginning at 18.00 used at Ebay , or 10.00 new for other names (always choose shipping and price lowest first from drop down menu)
Xubuntu should run well on what you have, but more ram would help.