If you dont want to make your phone thicker, you can always go for seidios oem sized 1650 mah battery (
http://cgi.ebay.com/647-Seidio-1650m...QQcmdZViewItem), but you'll only get about 10% more battery life. If you decide on a true extended battery, you will need to go for a decent brand battery like Audiovox, those cheap chinese batteries dont last anywhere near what they advertise, for a decent price id recomend one of these 2100 mah batteries for $29.99. (
http://cgi.ebay.com/OEM-NEW-Audiovox...QQcmdZViewItem)