You don't need to get an OEM BMW battery...you can get an Optima Redtop for about $100 no problem. People have even been getting Duralast and some other brands as well...just check out e46fanatics.com and do a search.
Toyota Tacoma Install
BMW 325Ci Install - Old
ZOTAC H55-ITX, Intel i3 & 320 Series SSD
7" Lilliput 669GL, M4-ATX, SB X-Fi, XMPCR
Win 7, RR, Garmin, DashCmd
Morel Elate 9 3way & IDQ10 ported (active 4-way),
JL Audio HD Amps, Audison BitOne.1