Nvidia's current offerings in that range arent very promising, their GTX285 perform similarly to a 5850, produces more heat, uses more power, and often costs more too. The GTX295 performs slightly better than a 5870 which generating a LOT more heat, using significantly more power, and sometimes suffering from crashes at the highest settings and resolutions with its 768MB frame buffer compared to the 5870's 1GB.
nVidia's Fermi based cards are due out eventually(supposedly march, but they may be launched in conjunction with duke nukem forever) but until they arrive ATI has the best options for the higher price ranges unless you are really really attached to physx.