Look closely at their website... they have a specific topic in their FAQ section which tells you which chipset/motherboards WILL NOT WORK with their soundcards. Generally, this is with cheap chipsets (which are causing problems with ALL soundcard manufacturers.)
As for specs, any modern PC will work. I'm using a lowly PII-450 with a Mia and an SBLive and have had great success. I've run over 30 16-bit tracks and by normal songs are 8-16 24-bit tracks.
Audio and video are very demanding, high data flow applications and you should NOT use cheap components if you are build-your-own. Trying to use a $399 PC is like trying to race at Daytona with a VW Beetle..... you might make the first turn, but the crashes you'll have will be spectacular and NOT pleasant!