You can buy an OEM, or A seidio extended battery.
Look in the CB store, there is an OEM sized battery, that holds a little more juice, and a double sized battery that comes with a larger battery door.
I also have a few OEM batteries, so if you are just looking to replace with an OEM, just pm me and you can send me a prepaid envelope, and i will give you one for nothing, i have 6 batteries for my Curve. (don't ask)lol
I bought a few back covers, and the guy gave me 5 batteries as well.
Yes OEM is Original Equipment, Seidio has a battery that fits under the Factory Curve cover, if i remember correctly, its 30-40% more capacity.
The large extended battery from Seidio, is 130-140% larger, but need a bulky door.