I m just building a seprate Rig ... for myself!
Well then, if you're happy with the performance of the HD7850, there's no need to spend more. If you aren't, then look at HD7870 < HD7950 < GTX670 in order of ascending power. Just be aware that the AMD cards won't offer PhysX, and the compute performance of Kepler is gimped (for things like folding or bitcoin mining).
If you'd be happy with less (oh the horrors! You might even save some money!) consider the GTX560Ti (unless you play Skyrim) or HD6870.