Well, i have been doing more tests during the last weeks, swapping in & out the cards, and i found the 480 to be faster in some scenarios, but slower on others. Overall, the 480 might be faster, but, i'm not happy with a card that is faster only sometimes.
According to my custom tests, my 285 is around a 15-25% faster when the application is bound by the texture sampling units, and the 480 is around a 40% faster than the 285 when the application is bound by shading power.
So, this is complicated because, to run 1920x1200 with high level of AF, you really need a lot of texture power, because you need to filter more texture samples, but also, you need the shading power, so, the decision is not easy.
I have been reading about the differences between the 480 & the 580, and it seems that i might be right, because nVidia increased the texture filtering performance in the 580, as i can read @ anand or xbit:
Personally, i'm going to keep using my 285, and i'm going to consider this 480 as my little mistake.
And if somebody wants a cheap 480, it's on sale