CrossfireX Mobo

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Crimsonator

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Apr 2, 2011
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Hey guys, I know this is one of those NOT ANOTHER ONE OF THOSE threads, but I really haven't looked into this too much, so I thought I'd consult the experts.

What should I be looking for in a motherboard for a dual-GPU setup?

Thanks!
 
Are you building a computer from scratch?

What processor do you have in mind?
 
Basically, I just need to know:

1. Should I look for a mobo with extra space between the PCIe slots?
2. What is the functional difference between PCI x1, x4, x8, x16, and x32? (more specifically the motherboards that say the PCIe slot runs at x16 while alone and x8 with another card plugged in)
 
Crimsonator said:
1. Should I look for a mobo with extra space between the PCIe slots?
Click to expand...

It depends on the cards you're going to run, and what other cards you also need to install. Most cards with dual-slot coolers will also occupy the slot directly below the card and will also like having a 2nd slot below the card free as well so that the fan intake is unrestricted.

2. What is the functional difference between PCI x1, x4, x8, x16, and x32? (more specifically the motherboards that say the PCIe slot runs at x16 while alone and x8 with another card plugged in)
Click to expand...

Generally there will be a maximum number of PCIe lanes as dictated by the chipset. With only one card installed it can afford to allocate 16 lanes to one slot, but with two cards installed it would have to split those lanes to 8 each (in most cases).

For the most part, if you are talking about a modern board with PCIe 2.0 slots, anything 4x or above shouldn't be much of a bottleneck. 8x would be safer, but benchmarks show hardly any loss using a 4x slot. Some boards offer gimmicks to be able to claim 16x on both slots, using things like an NF200 chip, but in almost all cases this offers no advantage and there are often even downsides (extra heat, stability issues with the NF200, etc).
 
GotNoRice said:
It depends on the cards you're going to run, and what other cards you also need to install. Most cards with dual-slot coolers will also occupy the slot directly below the card and will also like having a 2nd slot below the card free as well so that the fan intake is unrestricted.



Generally there will be a maximum number of PCIe lanes as dictated by the chipset. With only one card installed it can afford to allocate 16 lanes to one slot, but with two cards installed it would have to split those lanes to 8 each (in most cases).

For the most part, if you are talking about a modern board with PCIe 2.0 slots, anything 4x or above shouldn't be much of a bottleneck. 8x would be safer, but benchmarks show hardly any loss using a 4x slot. Some boards offer gimmicks to be able to claim 16x on both slots, using things like an NF200 chip, but in almost all cases this offers no advantage and there are often even downsides (extra heat, stability issues with the NF200, etc).
Click to expand...

Thank you!
 
Crimsonator said:
2. What is the functional difference between PCI x1, x4, x8, x16, and x32? (more specifically the motherboards that say the PCIe slot runs at x16 while alone and x8 with another card plugged in)
Click to expand...

16x/4x Crossfire X is about 5% slower than 8x/8x Crossfire X. The actual speed hit varies based on the application, however if you are concerned with getting the most out of your cards, 8x/8x is the way to go.

There is close to 0 improvement moving from 8x/8x to 16x/16x.
 
Will the ASUS 6950 2GB fit comfortably (or even fit at all) in a CrossfireX setup on a P8P67 Deluxe?

The reason I'm asking is because it is a three-slot card, rather than a two.
 
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Without looking at feature sets and solely looking at PCIe slot placement, eVGA Classified series would fit the bill. eVGA's x58 Classified 4 Way SLI board has 7 PCIe slots. Place first card in slot 1 and second card in slot 5. Be aware that you will have to have an eATX case for the board to fit. Standard ATX cases won't work.

http://www.evga.com/products/moreinfo.asp?pn=170-BL-E762-A1

Crimsonator said:
Where would I look to find a motherboard that is specifically designed for extra space between the two PCI x16 slots?
Click to expand...
 
AceGoober said:
Yes, the Asus 6950 2GB will fit, but the bottom card will be blocking most of the air intake of the fans on the top card.

Here's [H]'s pics of the Asus P8P67 board for reference.

http://www.hardocp.com/article/2011/03/03/asus_p8p67_deluxe_motherboard_review/
Click to expand...

There's really not very many pics out there of p67 boards with two cards in crossfire, so it's difficult to visualize what it would look like.

it looks like on the Asus and Gigabyte boards that there are two pci slots in between the 16x slots.

That being said, if you put two 6950s in crossfire, there should theoretically be one slot worth of room between the two cards since the 6950s are two slot cards, right?

Couldn't you put in a one slot pci cooler to help that top card then?

I'm building a p67 setup very soon and plan to do 6950 crossfire, so this thread is definitely pertinent. Anyone have pics of a setup they've made for reference? Thanks in advance.

**EDIT**
I forgot to mention that I game on a 120hz 22" monitor running native 1680x1050. I'm considering crossfire in the first place because I can do two 6950s modded to 6970s in crossfire for about the same price as one 580 with much more performance. I'm building this rig especially for Battlefield 3, so that begs the question, at that resolution, is crossfire with 4gb of memory much overkill? I really just want max FPS to get ~120 fps on any game so my monitor is in sync with my cards. Just thought I'd throw that out there so you're familiar with my situation, and might be able to comment.
 
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