Battery Info Update
Please note that the Duralast 49DL will NOT fit in an e46 convertible. Unlike the sedan, the battery tray will not accomodate a battery longer than the BMW OEM battery. The Duralast 94R fits perfectly, but at about $120 is much more expensive than the 49DL. I guess they figure BMW owners can spend more money.
Also, some people thought that the convertible had a special battery type due to the extra vibrations that convertibles endure. I can tell you that the OEM battery is the same part number for convertible and sedan. The extra shock absorption is done via the tray.