Right the fastest GPU on the planet at the moment is the 4870x2 its, two 4870 on one card - it blows the 280GTX out of the water!
BUT this does all depend on what size your monitor is - you are not going to notice a huge difference between the 260GTX and the 280GTX unless you go over 1680x1050.
I must strongly advise against the 280GTX as it severly overpriced, so you can either opt for the ATI 4870x2 or wait until Nvidia's next offering.