The GTX560Ti will be approx 50% better than the 280. Yet it also a much better overclocker, quieter, and uses less energy. It also brings DX11 support, and as mentioned on board HDMI (it works well, I used it). The GTX280 can still play any game out there in 1080p, but games like BF3 have to be set to medium and some low settings. In comparison, the 560Ti can play BF3 will mostly high and some ultra settings. The negative to the 560Ti is I could not give you warranty support.
The 280 was the top card 3-4 years ago, so it's still a solid performer. Yet is a little noisy and power hungry (probably uses about 50 more watts than the 560Ti). The 560Ti on the other hand is a current gen card and brings the benefits I mentioned.