Which PCI-E slot for soundcard?

I am building my first ever rig tomorrow and I have the Asus P8P67 Deluxe motherboard and the Asus Xonar HDAV 1.3 Deluxe soundcard. Obviously the GPU is going to take up the first PCI-E slot which is x16 bandwidth, so which slot would then be best for the soundcard?

It should go into the Pci-e 1x slot not the 8x or 16x slots

Thanks for the reply. Any particular reason? Is it because soundcards don't get any sort of improvement from added bandwidth?