Well it mostly will depend on what you will be using the rig for. If it is gaming a GTX 670 or better would work. But that also depends on the PSU you have as to weather it would be adequate to push the GPU.
It would be most helpful to know what you are going to be using/playing the most with your system.
Im thinking of getting a 450 watt PSU and im gonna use it for gaming and video editing well the kind of games im gonna play are like hitman absolution, gta4, fifa etc.
Well that gtx 670 is waaaaaaaaaaaaay much much much much tooo costly.....in my country I can get a ps3 with that much money..
so what is the bill and do you have preferred sites for shopping?
If the money are restricted I'd suggest, 7750, 7770, gtx 750, gtx 750TI. whatever fits you.
All are decent cards, just do not expect wonders - they are like basic gamer cards.