Their is only one ebay battery i would buy its made by one of the expensive Lion (play on Li-ion) manufacturers using japanese cells, the battery is a few bucks more expensive but it really is worth it as their is a lot more then mah to a batteries true strength. here is a little review i wrote based on a more detailed review i read and my usage of the battery.
http://forum.ppcgeeks.com/showthread.php?t=29170