WOW! I saw some articles about Advent Rising in gaming magazines and it looks AMAZING! It's basically like an action/RPG. You can develop your character depending on how you go through the game. Also, you have to make choices throughout the game that affect the ending - and the sequels!
Xbox or PC, I have to get it. The only reason it's not coming out on PS2 is that the PS2 wouldn't be able to handle this graphically intense game built on the Unreal Engine.