If its a 3G it will automatically do 4.2.1 because that is the most current software for that phone.
I've had more success with IREB doing a 4.1 restore however..
Go to http://www.felixbruns.de/iPod/firmware/
Click the drop down for iPhone/iPod touch/iPad.
Find iPhone 3G (4.1/8b117).
Download that and let me know when it's finished. ( it could take a little bit depending on your Internet connection)
I've had more success with IREB doing a 4.1 restore however..
Go to http://www.felixbruns.de/iPod/firmware/
Click the drop down for iPhone/iPod touch/iPad.
Find iPhone 3G (4.1/8b117).
Download that and let me know when it's finished. ( it could take a little bit depending on your Internet connection)