To answer your questions:
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What about the voltage? Can i put it straight at 220V or i need a converter?
Well, no you dont need a power converter, as far as i know the PS3 will work fine off 220v. Ive had my PS3 plugged into 220v for a bit over 2 weeks and it hasnt skipped a beat.
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I don't have any games ,so i need to buy one also from America! Which one gives the most of the power of the console and worth its money?
You can buy games from america or Japan, becuase there is no region coding on the games (yet). I would probably suggest you get Resistance, as its one of the better games out at the moment.
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Will it be Region free for the european games?
The PS3 will hopefully be region free for Europe, but noones 100% sure on that one.
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Can i play ps2 games from Greece on it?
You wont be able to play European PS2 games or DVD's on an american PS3, because of region coding.
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Is there ayone experienced that can say If it is truly better than an xbox360 (on graphics... let's compare call of duty 3 for both consoles)?
Ummmm no. The game with the best graphics (that i have played) was resistance, and I would say the graphics are about the same a Perfect Dark Zero.
But considering the PS3 has only just been launched, none of the developers have figured out how to use the hardware to its full potential. So the games will probably get better and better (hopefully)
Hope that helps