640 ram might actually kill your battery life while its sleeping, the more data that has to be preserved in memory, the more power it uses of thr battery.
What I do to save battery:
1. Networking Offf : Set a location that has no active network ports. Airport drains sucks like a b*tch...
2. Sleep - Shutting down, starting up may take 1-3% of the battery, sleeping usually doesnt use anymore than that in a matter of hours.
3. Brightness low - Get it as low as possible while you can still see, and if you're not looking at it, turn the screen off.
4. Dont use CDs - CDs usually take a great deal of power to spin around, rip images to your Hard Disk and run off there.
5. Quit as many programs as you can = yea, its picky... and annoying, but conserve clock cycles...
thats what I do...