HD 4870 > HD 5770 > HD 5750, but i recommend you going with 5770 because its runs cooler and quieter than 4870 and has less power consumption,also it packs newer techs like DX11,Eyfinity and the difference isn't huge.
and as for CPU,I think you meant Phenom II X4 965,if so then its faster than a X4 630
And your English is fine
No,because memory isn't the only important factor,5770 is overall a faster card(even if its 512MB vs 1GB 5750)
Well it seems the OP can afford the 5770 512MB and since the difference between 1GB and 512 isn't much,then going with X4 945 and 5770 512MB would be a good choice.
I dunno Maz. He definitely will want 4gb but if he can add another 2gb in the near future I think the HD5770 1GB would probably be the better choice considering he will be at 1920x1080 and you can upgrade the system ram but you can't upgrade the card ram.
How long would it take you to buy another 2 gigs of ram if you just get 2 now?