i have tried the iclarified guide on jailbreak my iPhone 2G cos i have the same trouble ( try to upgrade then my iphone goes to emergency mode) ...
after this step :

Step Eleven
Be ready to follow directions now. It will ask you to hold the home button and the power button for 10 seconds. Then, you will have to release the power button and hold the home button for 10 seconds to enter DFU.

it said succesful but my itunes says something about cannot restore then now my iphone cannot start at all....... try to ON cannot....
HELP MEEEEE......
i'm so devastated rite now....