***Update - it works but it is a hair thicker and it is difficult to snap the back case on***[/QUOTE]
Just wanted to mention that I did the same thing and used the battery from the other phone. It didn't seem to fit, but all you have to do is modify the plastics on the battery slightly, I just hacked at it a bit with a knife (yes, I was careful as I know batteries are dangerous creatures).
I bought an extended battery (look on amazon) by a company called "e-cell" and it claims to be around 2300mAh or so. Thus far it appears to have a very good battery life, very impressed with it - even when playing loads of games and also playing music.
Also, I wanted to thank you guys, your suggestions have really helped me improve this phone even more.
Now on with the micro SD card hack...