Purchase a 1600mAH extended battery:
Seidio Innocell 1600mAh Slim Extended Life Battery for Samsung Epic 4G / Fascinate / Vibrant
$49 and keep the same size slim form factor with stock battery cover...
or you could get the 3000mAH extended battery:
seidioonline.com
$69.95 and turn your phone into a size of a brick...Imo I don't think this is neccessary considering the DK28 rom I think the 1600 would be the best buy. Because the back cover just looks ugly. But I still do recommend it to those who won't be anywhere near power for a while.. Such as camping...or hiking.. sure you need it. Also it adds a rugged casing to help it withstand minor shocks.