The first thing i wanted to do to my CrazyFlie was upgrade the Lipo.
I have an rc planes/helicopters background so had a good idea of what to go for.
I chose a Hyperion 240mah as Hyperion are incredibly good quality.
The battery i use to get 9 mins of flight time is below...
http://www.hyperion-world.com/products/ ... 25-0240-1S
... which you can buy at various online websites.
I soldered new connectors onto the tabs and was all set.
The new battery adds roughly 1.5 grams to the total weight.