Which New Battery?

[TPIT100]

Hello All,

I have a 2003 4.6is that has just turned 40k. I bought it last January with 30k. I have had to put in a new alternator. The old one burned up the bearings and welded itself solid. On 2 cold evenings since replacing the alternator, the battery has been too weak to start the X5. I am pretty sure the battery is the original battery and is ready to be replaced. Now for the question. Do I need to replace the battery with an OEM battery, or can I put in something else? I looked around the forums thinking this would be a topic much discussed and could not find much that dealt with replacement batteries.