Its not really that simple...
I haven't heard any grounches with the current issue distros that are based around either 2.4.18 and 2.4.19 and either the KT333 or KT400 chipsets that are the most popular ones that XP motherboards run on. The straight AMD chipsets also seem to be fully supported. Pentium boards, same deal, they just cost 2x as much
Not being able to get online is a matter of how you get online. Winmodem? Good luck... too many chipsets and too much unsupported goo, manufacturers not releasing specs: www.linmodems.org
has a decent HCL for that.
Cable modem or DSL over a network card? Nearly every NIC seems supported these days, although I have run a accross a few disapointments, anything based on the tulip or realtek 8139 chipsets are uber-supported, and above all, easy to find: like the Linksys LNE100-TX.