An FYI for anyone interested. Here is list of batteries for an E53 4.4i/4.6is/4.8is. I'll try to keep it up to date.

Notes:
CCA is Cold Cranking Amps. CA is Cranking Amps. RC is Reserve Capacity in minutes. L x W x H. L is Length. W is Width. H is Height. FR is Free Replacement period in months. PR is Pro-Rated Warranty period in months.
North American BMW E53 X5's, as far as I know, which includes my own, were outfitted with relabeled Douglas batteries. Apparently Douglas sold their battery division long ago to East Penn Manufacturing (near Reading, PA) which now sells batteries under the "DEKA" brand.

If anyone has information to add please let me know.

BMW OE
- (Douglas) P/N 61-21-8-381-762.
720 CCA 160 RC $144.50 retail
- Current replacement is an Exide battery P/N 61-21-7-586-962.
720 CCA 160 RC $199.99 retail

DEKA
- Exact Fit Model 649MF
900 CCA 185 RC 13.9375" x 6.875" x 7.5" 18 FR 70 PR
- Exact Fit Model 9AGM49 (All Glass Mat version of above)
900 CCA 180 RC 13.9375" x 6.875" x 7.5" 18 FR 70 PR

DURALAST
-Gold Duralast H8-DLG
760 CCA 100 RC 13.9375" x 6.875" x 7.5" 36 FR 96 PR

EXIDE
- Classic 49-60
850 CCA 1000 CA 140 RC 14" x 6.875" x 7.4375" 20 FR 60 PR
- Global Extreme L5/49-E108
900 CCA 1080 CA 185 RC 14" x 6.875" x 7.4375" 40 FR 108 PR

INTERSTATE
- Megatron Plus MTP-49/H8
720 CCA 900 CA 176 RC 13.9375" x 6.875" x 7.5" 30 FR 85 PR
- Megatron Plus MTP-49/H8A (All Glass Mat version of above)
900 CCA 1000 CA 158 RC 13.9375" x 6.875" x 7.5" 30 FR 85 PR

Links:

BMW: RealOEM.com * BMW E53 X5 4.4i Original BMW battery (Warranty only)

DEKA: Automotive Batteries: Deka's Complete Line of Lead-acid Batteries for Passenger Cars & LTV

DURALAST: Battery | 2005 BMW X5 AWD 8 Cylinders 4.4L EFI DOHC | AutoZone.com

EXIDE: Transportation NA

INTERSTATE: Interstate Batteries Category Page - Automotive / Truck BMW X5

Some good info: Consumer Reports Article - MSN Autos

And a great write-up on chargers: http://www.xoutpost.com/bmw-sav-foru...w-one-fyi.html