My point: Your a beginner (already established), You can make this game (true, but if you want it to be anything memorable, you might want to practise on some other "weaker" projects before you finish this game), You file size, even w/ streaming sound, will be prolly a lil over 10MB, depending on what you add in the game, if this "dream" game is what it's cooped up to be though, Tom will allow it through the portal. And if it's to big, Well get permission by the companies your using the sprites from and the companies who the players belong to, cut em some % of profit, and call of XBLA (although regardless, I don't think it will go that far...)
Well I honestly dont think it will be good enough to get through the portal (quality and graphic wise) I dont think the characters will be good. And you should practice before you jump in to a huge project. Maybe if you should us some of your work it will give us an idea of what it will look like.