[Galaxy S III I9305 (4G LTE + 2GB RAM) Android Deve] btemtd:
Nice work, Some people flash there phones before reading everything properly and brick there devices, But you did a very good move by doing NOTHING untill 100% sure. Because of this thread and my instructions bellow I decided create a thread (What to do after you buy the 4G S3) Either go there or just keep reading on here.......

So before you flash anything, or start playing with your gem, follow these important steps below to get all the important parts backed up and safe, so that you will not have to return your device back to samsung broken. Believe me I have seen it happen PLENTY of times in this forum, so read carefully :)

Make sure you have a Good USB cable and a stable PC and that don't get any power outages or surges, you dont want to be half way through something and you loosing power or bad connection.

1st: Install Samsung Kies on your PC this will then Install the Samsung USB drivers you will need to connect your device to your PC OR you can get the samsung drivers on there own from the Link in 2nd step, but I recommend you install kies as it may be useful to you in the future.

2nd: Now as your IMEI number is like the heart of your device lets back it up right now: Follow only part of these steps in this link, you will finish the rest of the backups at the end when your rooted. I have copied and bolded the parts you will need to do right now, once finished this section of the backup you should be left with a *.qcn file which is roughly around 250kb of size.

http://xdaforums.com/showpost.php?p=33048706&postcount=1

*Setting up QPST with your PC:*

*Backing Up IMEI / NV Data:*

*Restore USB Settings after Backup / Restore:*

Now keep that page open because you will return here in step 6 once you are rooted to complete the backup process.

3rd: Now Install Odin V3.04 - You can get the link for odin in the 4th step link, So you may aswell just continue on to 4th step as the instructions inside that link advice you to install Odin aswell.

4th: Now you must Root your device and install CWM recovery , Follow these steps carefully > http://xdaforums.com/showpost.php?p=32235061&postcount=1

5th: Once you have root I recommend you instantly make a full backup of your device also known as a Nandroid backup. You must go into recovery mode to do this with the CWM recovery you have just installed in 4th step above.

6th Now lets finish the backup process since we are now rooted, I am going to say this again, this is VERY important incase something goes wrong and you loose your Imei and Data, you need to back it all up. By doing this it will save you returning your device back to samsung like many have done previously because they have not done this, Remember where you left off in 2nd step above? Well now you can go back and complete the partition backup of the efs folders. You should be left with 4 backup files for this final section of the backup process >>> http://xdaforums.com/showpost.php?p=33048706&postcount=1

--------====FINISHED====--------

Congratulations you now have your Imei/Nv data backed up , You are rooted and Nandroided. Now you can play around but off-course DO NOT be reckless make sure you know what you are playing with

Peace

PS: Only trust what you see in this I9305 XDA forum, as i have been following the device from day one and I can only recommend the information you see here.