i'm debating about this too. my dealer quoted me $367 to replace and register the 90Ah 720CCA battery. I would do it myself and register it with some cables and available software you can find on the forum but I've read that some have found out that late 2006 and early 2007 cars can't be registered by bmw or aftermarket software. I wish I have more confirmation on this. Seems like quite a few 06 and 07 batteries have gone bad.
btw my dealer currently has a $65 off coupon so I might just bite the bullet.