I found it interesting to know that ADSL modem can be used via USB in GNU/Linux. I was told that its tough. Yes, indeed its tough. But its possible. For eg, my Siemens modem use GlobeSpan chipset. The USB driver for the same for GNU/Linux is available for download and appriciable work is being done in this direction. The configuration seemed a bit too geeky, but a small effort can help a user to setup the modem and use it via USB.
Well anyway, its always better to use modem via LAN card!!