The GTX 580 is slightly faster than the GTX 480, largely due to the 580's higher clock speed.
With my PPBM5 testing I did find about 1 second improvement in my 1.5 GB GTX 580 on the MPEG2-DVD test.because that PPBM test is used for the scaling from 1920 x 1080 to 720 x 480. With the Windows clock for our measurements being a 1 second clock it took many runs to verify that there was an improvement. So if you already have a GTX 480 it is really not worthwhile upgrading (i should always add the qualifier that this was 5.0.3 version and I do not know if they can change the coding for improvements in future versions). No experience here with the 3 GB version. I did then overclock the EVGA GTX 580 with their EVA Precision tool and even with all three clocks at the maximum found absolutely no improvement in that score.