The GTX 650ti will be very good for your resolution. It is faster overall than the AMD 6850.
http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_650_Ti_Direct_Cu_II/28.html
Be aware though that they both use more power than the 4670 (more so the 6850) & require a supplementary 6 pin power connector (or dual molex to 6 pin). if you need a low power consuming card that don't require supplementary 6 pin power, the AMD 7750 (GDDR5 model) or GTX 750 are good candidates if you can find one or budget it.
What make/model of PSU do you have ?
my specs are
intel core2duo E8400 @3ghz (running at stock cooler)
4gb ddr2 800mhz ram
1tb hdd
asus p5kvm motherboard
cooler master extreme power plus 460w (it has 2 pci-e 6 pin adapter and plenty more adapters)
i'm using this system for the past 6 years.
actually the card i'm buying is a used one the priginal price is double the current price in mentioned .also i'm experiencing coil whining in my psu asin my state we did not get enough voltage during night times.