There's no reason the DS can't handle a few thousand triangles and a half-decent amount of texture on one model in a viewer and run it just fine.  I had some Half-Life 2 models on the DS, all the texture detail gets lost (no filtering) but the meshes themselves worked and performed just fine.  I wish someone would make a 3D fighting game (Mace: The Dark Age DS) and then could probably use such polycounts in a proper working game.  HL2 is obviously out of the question but a fighting game with only a couple of character models could really squeeze some impressive graphics out of it that would be HL2-like.  Yet, we have none.