4 GB is the biggest memory card guaranteed to work. There is conflicting info about larger cards like the 6 GB working and lots of speculation on what will be supported in the future. But for now the 4 GB ones are pretty big and definitely work.
As for the backlight timeout, 30 seconds is the default. You can change it in Options > Screen/Keyboard > Backlight Timeout. You can change it from 10 seconds to 2 minutes, but your battery life will probably take a hit if you increase it much.