Is there any point of changing Graphics to old high end card : Saphire 4870x2 ( i can get this one for just a half ebay price )
At least I want to know what kind of PSU power and brand i need? for e.g. corsair 600W is enough? or mid brand 750W ?
Just wanted to play battlefield3 normaly ( for e.g. on medium at least at 70fps or higher settings 40 fps ? ) asasins creed and other new games.
It would be good upgrade ? What should I do
If that's whats in your budget, I would say it's probably a good upgrade and it will run well. I have an old PC I donated to my brother that had a Phenom II 555BE and Sapphire Radeon HD 4850X2 and it ran well on an Antec 550W PSU. He still can play modern games at respectable settings and resolutions without FPS dropping below 30. StarCraft 2, BF3, Skyrim, Mass Effect 3 and MW3 all run at a mixture of High/Ultra (with 2X AA, so the GDDR5 on the 4870X2 will help increase it if you want) at 1920X1080. It's a good idea to maybe spend more and reach a little bit on that PSU and get one in the 700W-800W range because you can always upgrade to a 6970 when they drop a bit more in price and maybe even look for a Thurban based CPU for cheap second hand.