I am planning to root my Xperia Arc S and want to know whether will I be able to upgrade it to Android Jelly Bean.
Sony stopped with ICS, it didnt release Jellybean for Arc S. If you are rooted and installed CWM recovery you can install Jellybean Custom rom. There are plenty of rom available here. Just read the instruction before installing the roms. There are roms which are only for unlocked bootloader.
Most of the times, if not all, rooting is necessary to upgrade to a newer version, especially if it is unofficial(custom ROM). You also have much more power on your phone, but it has the drawback that the guarantee will be voided.
In your case, I have found that it is easier to upgrade. You just need to install pc-companion and after you connect the device on it, it will download the latest update(Jelly Bean).
If I were you, I would download the update first and then I would root my phone.