well,Geforce GTX 460 1 GB would be the option for you if you are looking for nvidia card.at stock,it is slightly slower than HD 6850 in most games but once overclocked,you will get good performance boost.not to mention you can do the same with HD 6850.
I personally would not recommend GTX 460 with your CX430 power supply as it takes pair of 6 pin PCI-E connectors and power consumption is also high.HD 6850 requires single PCI-E connector and you can safely run it with your power supply.for GTX 460,I personally recommend a 500 watts of power supply with two 6 pin PCI-E connector like corsair CX500V2.
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