I recomend going with the KN8, I have used a similar motherboard from Asus and am satisfied with its delivery. The chipsets wont make much of a difference, but the KN8 also has a gigabit lan while the RX480 has only 10/100. The price are about the same unless you are planing on getting rid of this system in a couple of months, get the KN8.
I have a board with the xpress 200 chipset. It's not bad, but the SB400 wasn't up to par compared to the nforce chipsets. I would stick with the nforce. Other boards with the later South Bridge revisions are much better performance wise. The main dificulties are in USB support and the management of IDE/SATA data rates (slower compared to nforce 4, especially the USB).