Most likely it will work. There have been some rare cases where a PCI-e 2.1 card does not work in a PCI-e 1.1 slot.
Some people say it is because the PCI-e 1.0 or PCI-e 1.1 slot was not implemented properly on motherboard by the manufacturer. It is difficult which motherboards have this issue unless someone has already posted about it.
As I said, this issue is pretty rare. You may want to go to Asus' website to look for an updated BIOS that addresses this issue... if it exists and if Asus bothered to correct it.