VERY IMPORTANT, ACTUALLY MOST IMPORTANT THING TO DO:
As soon as you're rooted, get Voodoo OTA rootkeeper. its free from the marketplace. then open that app up and Save and backup your root. REPEAT, SAVE & BACKUP YOUR ROOT. you will kick yourself in da a$$ if you don't..lol. then now you can update to the latest firmware for 300, which is .26 or whatever. Once you update, you will initially lose root. but no worries. you just open up the voodoo app and restore your root. Now you are rooted again on latest firmware. you will use this method from now one to restore root after a firmware update. there has already been confirmation of a 300 owner rooting on.17, then using voodoo to save/backup his root, then update to .26. then using voodoo to restore root. now he is rooted on latest .26 firmware for 300.
you will be sorry if you don't use voodoo to save your root. even if you don't immediately update afterwards. only reason root is working on 300 is because its the ones with .17 firmware. there is no root method currently available or even in developement yet, as far as we know, That will work on .26 or higher. SO BACKUP N SAVE YOUR ROOT FILE WITH VOODOO OTA keeper. do it immediately, don't wait. I've seen too many people gripe and cry about losing root because they forgot to backup n save root. now they are stuck on latest firmware and can't root. that was at first but now prime has downgrade methods that seem to work for some. but this hasn't been confirmed to work on 300 though since there is no blob file posted for .17.
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Prime/Tegra3 Gamers Thread