The Xtreme Audio is the worst mainstream sound card you can buy, in my opinion. Basically you have no EAX, slightly lower sound quality, and no modes to choose from. You just get the Crystalizer and CMSS-3D for audio purposes.
The XtremeGamer is basically a strip down version of the XtremeMusic (Base Model X-Fi card). It offers the same specs except it has limited ports compared to higher X-Fi models. It still offers EAX 5.0, all 3 modes, etc.
No one should ever consider the Xtreme Audio, it's not worth your hard earned money. An Audigy 4 SE is cheaper and performs better than the Xtreme Audio. The XtremeGamer is fine for those that game and don't need much I/O connectivity, but the XtremeMusic is close in price and has the extra ports.