shouldn't matter much :D

Any slot will do, but go into your bios and assign the irq for the slot you use manually. Go with an irq that has a higher priority. On Asus boards, typically 9 or 10 would be good numbers. Try to avoid irqs like 3,4,or 5 which have a very low priority. Save those numbers for things like your network card. Also, look in your bios for the "Pci Latecy Value" and see if you can turn it down to zero. Default is normally 32 clocks. Oh..and make sure you set "Plug n' Play O/S" to "NO". This has the effect of keeping windows from reassigning an interupt beyond what you've already specified in the bios.

I've found these tweaks to be especially helpful to getting my Motu to work properly on my P4T-E. And Motu cards are known for their very poor support on the pc.

Actually it does matter for certain cards...Delta cards like to be in slot 4 or 5 and actually work better with IRQ 5 or 10...
Certain cards are finicky as to when they talk to the processor so it "does" matter where you put the card. in fact any audio card should NEVER be placed on Slot 1..which shares with the AGP.
If need be you can assign the IRQ as Keith mentioned but try NOT to do that first. usually you want to install the sound card first before installing any NIC or modems...
Opus

P.S...certain cards DO NOT like the PCI latency to be adjusted...ie Echo Audio cards...MOTU is probably the exception tho...those cards are just plain weird in Windows based systems..especially non Intel based boards

What should PCI LATENCY be for echo cards?

Mine is currently set to 15 but I am not sure that is the default. I seem to remember maybe chaning it.

I am having problems with pops and clicks on my layla.

d./

Opus is correct. It CAN matter which slot you use, depending on the card. Which ever slot you use, you will optimize your audio system if it DOESN'T share an IRQ with another device.