As for you requirement and your processor I don't think that ATI's HD4850 is the right choice for you. bec it will be bottlenect due to the processor and certainly you need a 500W PSU.
So acc to me you should go for ATI's HD 4670 . It will be around 5.5k and you will not have to change the psu for it. Also performance wise its neck to neck with NVIDIA's 8800GT.It will surely rock the games at mid settings and for old ones no question at high settings.
http://www.guru3d.com/article/stalke...ance-roundup/5
And if you ever thinking of upgrading the processor which you should then you MUST buy HD4850 its unbeatable in its price.