easy, that is the 650ti BOOST or the HD 7850... the 7850 has more overclock headroom, and depending on the model should out perform the 650ti BOOST by a few percentage points in most models... but that said, they're really pretty close in their performance. Close enough that if you perfer Nvidia to AMD you won't notice the difference.
If you don't plan to SLi/xfire in the future Stick with the 1gb versions... these are both just slow enough you won't need 2gb of vram (if you do plan to xfire/SLi those cards, you should definately get the 2gb cards, that said, xfire is broken, so if SLi/xfire is in your future plans, get the 650ti BOOST, not the 7850)
The next step up in gpus are the 660 and the 7870... if you can find one on sale inside your price range hop on it.