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OK so this may be a foolish question since I'm completely new to multi-GPU.

I have a Gigabyte GA-990FXA-UD3 with (2x) 16x and (2x) x4 slots.

I have (2x) XFX 6870 and (1x) XFX 6670. 

Ideally I'd like to use the 6670 for desktop GFX and the two 6870s for LTC processing.

Is this possible, or would I need (3x) x16 slots?  The only reason I ask is because it appears that people are stringing additional GPUs from 1x slots, so maybe all lanes aren't needed for processing somehow?

Thanks!

let me get it straight:

dual gpu board on 1x pcie 1.0  = 1/2 * 1/16 * 1/2  = 1/64 pcie speed per gpu than a single gpu board on 16x pcie 2.1

no visible difference in hashing speed.

I don't understand what you're saying about my particular question but I'm seeing threads where people are running 7GPU from this board somehow - that's what I'm trying to figure out.

Here's an example of someone running A LOT of GPUs off my board:
https://bitcointalk.org/index.php?topic=94657.0

So it seems to be a possibility.  I just need to figure out what will be needed and which cards to put in the x16 slots and which to put in the x4 slot.

Right now I'm thinking the low-spec desktop graphics can go in the 4x and the two processing GPUs can go in the x16 slots.

basically you will need something called PCI-E extender, to convert any non 8x/16x pcie slots to accommodate your video cards.

how many video cards you can put on one mobo depends on how many pcie slots you have no mater if it's a 1x, 4x, 8x or 16x

if you have 2*16x and 2*1x, the maximum video cards you can put is 4, in terms of mining, 1x and 16x like xorxor said, I quote "no visible difference in hashing speed."

the picture you posted, the mobo has 2*16x and 4*1x, so it can hold 6 cards in one mobo.

Awesome, thanks man - that's just what I needed.