[praka123]

usb support for dsl modem are feeble(and i believe usb is unstable,cmprd to pppoe).there are some projects for supporting few chipsets.
for eg:eagleusb,eciadsl etc
reg support for old pc's.there is some debian project for this(forgot the name)
and if ur a little bend on getting a good soltn,you can install gentoo or debian without gui,and later select some less resource hungry window managers like ratpoison etc