it just needs to be compatible for the incredible....and it will say so on the add. the eris batteries should work in the incredible as well. it comes with a 1300mAh so you want to get a bettery that is at least that. i think you can find ones from 1300-1750 that will be the same size. if it's over 1750, it will prolly be thicker than the factory battery and the door wont fit over it.
http://cgi.ebay.com/2x-BATTERY-CHARGER-VERIZON-HTC-DROID-INCREDIBLE-/220621529067?cmd=ViewItem&pt=PDA_Accessories&hash=item335e115feb
this is what i got so now i have 3 batteries with an external charger. works out real well.