Like carrerarsr has mentioned:
644 is if you have a standard radio.
633 is if you have iDrive/Business/Pro Nav.
Both come with a snap-in adaptor holder under the armrest. If you want to have iPod control you'll also need the USB Interface option.
I have an iPhone snap-in adaptor for my iPhone, it keeps it charged and uses the roof external aerial... but that's it, the iPhone connects via bluetooth still.
For music I have a 30Gb iPod 4G classic, that i use a Y-adaptor and the USB interface... which is also under the armrest.
Hope this helps.
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