Linux brings it's own drivers. Some are directly compiled into the kernel, some need to be loaded as modules, depending on your distribution (I jumped out of Red Hat after 7.2, so I can't tell you exactly).
Almost all ethernet cards are supported. If you want to be on the safe side, buy one with a 3Com or Intel chipset (my personal preferences - many others like DLink or Realtek work well, too).
Red Hat has a hardware recognizing tool (Kudzu) being called during boot. You will be asked some questions on boot after you installed the card.
The device will then normally be eth0, ready to use.
For further configuration for DSL a good start is the DSL-Howto:
http://www.tldp.org/HOWTO/DSL-HOWTO/
For all big German providers there is also a great step-by-step website (
www.adsl4linux.de) - maybe there exists something similar for your country.