Crossfire question for GA-P55-UD4P

[SquadBumrush]

Hi gang!

I've read on these forums that there should be no problems running a single GPU setup in the x8 PCIe slot (as opposed to the x16 slot). Sure, there may be bandwidth concerns, but that's not exactly what I'm asking.

I current have an Asus EAH5850 with an aftermarket cooler that causes the total footprint of the card to occupy 4 expansion slots. This is no problem with a single GPU setup, as there is plenty of space and I have no other PCI cards in the system. However, the sheer size of the cooler now prohibits me from running a multi-GPU setup so long as the card with the cooler occupies the x16 PCIe slot (since there are only 2 slots of empty space between the x16 slot and the x8 slot).

My question is: if I run a 2-GPU CrossFire setup, is it possible for me to assign the card in the x8 slot as the primary? (EDIT: yes; CCC should provide a means for assigning one card as the Display GPU, thus assigning the second card as the "Render" GPU.) I play some games that don't support CrossFire and I'd prefer to disable CrossFire in those instances. The trick is that the card I'd prefer to use is the one with the aftermarket cooler, which as I mention above can only be positioned in the x8 slot if I'm going to run CrossFire. (I don't know if this is a question for Gigabyte or AMD.)

Let me know if you need any further clarification and thanks for your help!

EDIT: after browsing the Catalyst Control Center Help docs, I may have discovered the answer to my own question. Per CCC:

"Card Reversal Recommended

A recommendation message to reverse the graphics card positions may appear if the primary graphics card is not installed into the primary PCIe slot. Although AMD CrossFire™ can be enabled, performance may be improved by reversing the graphics cards in the computer.

See your motherboard’s manual to determine which is the primary PCIe slot."

Obviously, on my mobo the lone x16 slot is the primary. Perhaps the question now becomes how much performance will be impacted by the reduced bandwidth of the x8 slot? Most everything I've read online indicates that the real world difference between the x16 and x8 slots is negligible when gaming at low-to-mid resolutions. To clarify, I'm running at 1920x1080, which seems to be generally classified as "mid" or "mid-high". My EAH5850 is overclocked to 1010MHz core / 1245 MHz memory. Would anyone with firsthand experience care to share facts/opinions?

[Psycho101]

There is zero performance impact unless you run eyefinity resolutions of 3840x2160 or 5760x3240. Even then the difference is a couple of frames at most.

Search HardOCP, they've done 16x/16x, 16x/8x, 8x/8x and 8x/4x comparisons. Even 4 lane Crossfire isn't effected until moving to 1920x1080and beyond.

Also be aware that running two GPU's in your board will result in both slots running at 8x. Also note that with a multi GPU setup, full speed USB3 and SATA6G will both be disabled.

Edit: If you plan on getting another 5850 for crossfire there is zero advantage in setting your original card as the primary, even if its clockspeeds are faster etc. You will get identical results regardless of which card is the primary. The only time this matters is with hybrid crossfire. For example you can crossfire a HD4870 and a HD4850 together. In this case you would want the 4870 as the primary so that you can use it solo in games that don't scale with CF well or in new releases that currently don't have a CF profile.