The major thing to watch out for is not to get anything built for windows. Stay away from WinModems, WinSound Cards, Win Video Cards, etc...

Depending on which Unix you go for, support for most generic PC hardware is very good. Some new / bleeding edge stuff may not work fully yet, like some of those tricked-out video cards.

For the most part though, any basic PC will come up under Linux / BSD, although Linux seems to have better support for cutting / bleeding edge stuff.

Also, if you check out places like www.redhat.com or www.linux.org , you'll usually have a hardware compatibility list, if you really want to be sure.