The easiest way is with z4root app. It's not in the market any more but the apk is easy to find. I used it to root and it worked flawlessly. I haven't had to unroot yet though. It's worth a try.
It depends on what you have changed on your phone. The update will check to see if the required specs are present. If not the update will not load. If you were running a custom rom and flashed back to a stock backup, you would not have to unroot to update. You would likely have to root again after the update though.
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