Sabertooth 990 FX PCI-E speed confusion

[spadesvet11b]

I have a great question concerning the PCI-E slots, and I hope someone has the answer.

Here is my rig-
ASUS Sabertooth 990FX
AMD FX 8120
G. Skill 14900 1866 DDR3 (8 GB)
SLI-2 X MSI GTX 580 Lightning extreme
Intel 520 120GB SSD

Ok, I have my 580's in PCI-E x16 slots 1 & 3; they are covering my my lone PCI-E x1 slot and my PCI slot. The only available slots that I have for expansion are my black PCI-E x16(x4) slot, and the last PCI-E x16 slot, which is brown. I NEED to get a sound card; the onboard is garbage, but here is my issue. If I sandwich a sound card between my 580's into the PCI-E x16(x4) black slot there is going to be some serious heat issues, and, where I am confused is the last PCI-E x16 slot. If I install a sound card in the last slot what I am understanding is that it will degrade my speeds from- X16, X16, 0 to X16, X8, X8, which I don't want to do. I am not sure if the x16 x8 x8 applies only for 3 way SLI or if it is any card that is put into that final slot.

So, I am hoping that someone can help me out with this issue.

 

[RGone]

As I understand it, it will be anything plugged into that slot. The switcher is only looking for "something" not video card plugged in the slot.

 

[Neuromancer]

If you plug into brown slot you will drop to 8x 8x. on your 580s.

You will have to use the black slot and find a card short enough that you only block one fan on the top 580. then add a side panel fan to bring cool air in to counteract the loss in airflow.

Actually sound card and dual reference 580s would be totally unaffected thats a shame. Do you bench is that why you bought the lightnings? Or just have them both clocked well past 900?

EDIT: I just dont see any motherboard providing what you need with out sacrificing tri-fire/SLI compatability. Which no manufacturer would do apparently unless limited to 16x lanes total

 

[Mario1]

You won't have a big performance loss if you run them at x8/x8.
Cards can't utilize the full potential of x16 lanes anyways..