This is the well explained procedure (with huge pictures) I followed to replace my iPhone 3G battery with a new one (1600 mah).
I payed less than 10€ for the original battery (shipped at home) and it took me 25 minutes to replace it. after 12 hrs of standby plus some Internet connections (I checked my email three or four times), the battery charge is still 85%.
CHALLENGED ACCEPTED.
GOAL ACHIEVED
LIKE A BOSS.