I bought a new Samsung galaxy s Advance(GT-i9070)It was came with android Ginger bread.It was actually a phone with 768 RAM-([enter link description here][1])I clearly verified this from shop when i bought this.Last day i upgrade my phone to Jelly bean with official Russian firmware and reset to factory default settings.But now the phone seems very slow .also full RAM Capacity seems 521 MB at Application manager.All applications are now consuming  memory from total 521 MB memory.
My question is.
Where is remaining memory? How i can obtain remaining memory to speed my phone?Some people on web says to root the phone,Install ROM manager etc.Does these approaches good?Please give me a solution


  [1]: http://www.gsmarena.com/samsung_i9070_galaxy_s_advance-4469.php