I think it was on Cartoon Network. It was kinda like Yu-Gi-Oh!, but it took place in a battle dome. The kids would go and fight tournaments with their monster cards and the arena changed (i.e. jungle, quarry, etc.)
Probably Duel Masters?
According to Wikipedia, it aired on Cartoon Network.
It made a truncated three-episode preview premiere on Cartoon Network's Toonami block on February 27, 2004. The series then made its official premiere on April 13, 2004, as part of the Saturday Video Entertainment System block, with fewer edits. When Toonami was moved to Saturdays, the block premiered the rest of the series.