All you need know is for full linux compatibility is:
No ****ty soundcards (creative have the best *nix support, period)
No ****ty videocards (nVidia have the best *nix support, period)
No USB or internal software modems - they almost never have ANY non-Windows support
There is sometimes a bit of trouble with less commonly used accessories such as firewire and other exotic stuff, but like I said - just go to the site of your distro of choice, key in the hardware you're thinking of getting and it'll tell you whether it works or not.
If you want tips for hardware, tell us what kinda server you're going to be running (namely file, web, mail or the works?).
Spread the Tux love.
"Would you buy a car with the bonnet* welded shut?"
-choice quote form the RedHat Linux 7.3 installer
*commonly known as a hood to all you crazy yankees