Anyone know if it's possible to connect an MP3 to the 330xi stereo system via an adapter? And/or is there an MP3 stereo upgrade out there? Thanks.

yes, theres lots if you dont want to change your head unit ,they have some that will play right through your radio signal ,kinda like an fm modulator for some cd changers.

Any head units that you'd recommend? Any that don't void the BMW warranty?

I dont know about bmw warenty but most cd/mp3 decks have there own warentys you can even get extended plans.If your concerned about the deck causing a problem with the cars electrical it wont ,as long you get it installed by a profesional,hope that helps.

thanks