Hi folks, I just installed two Radeon 6870s on a mATX Gigabyte GA-Z68MA-D2H-B3.
While the MB officially "supports" SLI and Crossfire and has four PCI 2.0/3.0 slots running at x16, x1, x4, and x8 (in that order, left to right), you can only plug in the cards in the two slots (x16/x4) that "support" dual GPUs and that can "fit" in the case since those cards+coolers are pretty big and hog up both slots.
1. I noticed on GPUz - even with full-screen/power my setup runs at x16/x1 instead of x16/x4. I searched all over for this and couldn't find out why... is it incorrectly reported? Is it because I'm using all 6 SATA devices? Any other ideas? I assume that x1 card is causing quite the bottleneck?
2. That being said, any recommendations on mATX boards (socket 1155) with x8/x8 or even x16/x16 SLI support?
According to the specs for that board, the x16 and x8 slots should be the ones you use for graphic cards. They will each default to x8 speed when both are poplulated. Unfortunately, on a mATX board, installing a card in the x8 slot will mean a large card will cover the motherboard's wiring headers. That may cause a problem with the physical wiring. http://www.gigabyte.us/products/product-page.aspx?pid=3...
If you are not using the x16 and x8 slot, you are limiting your 2nd card's potential. In fact, from my experience, even the x4 slot will drag down performance of both cards. They need at least x8,x8. Depending on how much you can afford to spend, here is a mATX board with x8 x8: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
But your board already has x8,x8 capability.
Your motherboard already supports x8/x8 if you use the correct slots. The reason why the PCIX8 slot was placed as the fourth slot is so that the second graphics card wouldn't obstruct access to the SATA connectors. That's what happens when the motherboard cheaps out and doesn't use right-angle SATA connectors along the back edge of the motherboard.
Micro-ATX boards are the worst motherboard form factor for dual graphics card systems because their limited amount of board space forces poor onboard I/O port layout. You need to allow an empty slot width of space between the cooler of the primary graphics card and the back of the secondary graphics card to ensure that the primary graphics card gets enough cooling air. If you were liquid cooling the two graphics cards then Micro-ATX is fine.
Okay, I hear you guys and maybe I'm being a total idiot here... but I don't see how you can physically fit two Radeon 6870 cards in the GA-Z68MA-D2H-B3 board's 16x and 8x electrical slots. The layout of the MB physically, from the middle to the edge of the board, is x16, x1, x16, x16... and electrically x16, x1, x4, x8.
These Radeon cards and their HSF/plastic shroud both occupy two "slots" on the motherboard and the back of your case. Isn't the only way to plug in these cards by using the x16 and x4 slots? Or do people "inverse" the mounting bracket that screws into the case to physically use the 1st and 4th PCIe slots?
Oh, and guys... I think I'm slowly finding out the answer to my own question... it's been a combination of your great answers here and scouring the web for fragments of info. I found a review of a similar (but not exact) Gigabyte mATX board that summarized my problem:
"There is one additional limitation of this slot layout. Radeon cards are able to Crossfire in x16 + x4 mode with a tiny performance hit, but GeForce cards can only (officially) SLI in x8 + x8 mode. Thus, for SLI or optimum Crossfire peformance you will need to somehow utilize the first and fourth PCI Express slots, not the more common first and third. Most micro ATX cases support only 4 slots. Thus, the lowest slot cannot be a dual slot card, severely limiting your dual graphics options. There are a handful of enthusiast micro ATX cases which supports five expansion slots, but those are rare. Also, the coolers on some dual slot cards will interfere with cables plugged into the headers on the bottom edge of the motherboard."
So yes; given how Gigabyte has put the x16 and x8 (electric) PCIe slots as the 1st and 4th slots of their MB, it's very hard to use these slots for a Crossfire setup because you can only fit them in the 1st and 3rd slots.
Bottom line = looks like mATX isn't the best option for Crossfire/SLI unless you want a highly specialized board. I'll look at those boards and also consider moving back to regular ATX.
You would, in that case, be advised to change cases or motherboards. Unless your present case is very expensive and you don't want to lose it, changing cases would be the way to go. Btw, I disgree with the statement "Radeon cards are able to Crossfire in x16 + x4 mode with a tiny performance hit, but GeForce cards can only (officially) SLI in x8 + x8 mode". The biggest performance hit will be x16 +x4. In fact, X8+X8 will provide more than adequate bandwidth for all but today's most expensive cards. Most cards can't even saturate X8.
Just a thought but i accually had a similer issue with gpu fan space, i ended up taking the heatsink off of both my radeon hd 5770s and tossed on TINY replacements from the sapphire hd 6570, under full OC load with crap fan/air management with antiquated parts i never touch anything over 68C experement with different fans most have indentical screw holes and useable connecters to the card itsself.