How do I know if I need throughput of PCI-e 3.0 x8 vs. x16?

[Kluge]

I plan on purchasing a few 5850s and putting them in Crossfire.

Will I see performance degradation if the cards are in PCI-e 3.0 x8? Would x4 work?

5850s came out something like a year before the 3.0 standard, so I can't imagine they're bottlenecked by 2.0 speeds which are essentially 3.0 x8 speeds, I believe, but I would very much appreciate if someone has experimental data on this.

And while you're here... anyone have a suggestion on a <$100 AM2+/AM3 mobo to purchase which would support two 5850s and preferably give them some room to breathe? I'm considering the ASRock 870 EXTREME3 @ $90 shipped.

Thanks.

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[Convery]

For GPGPU work (mining) you don't need much bandwidth so a PCIe x1 or even a PCI 23 bit would work. Though if you are going to game on it you need at least a PCIe x8 slot but preferably a x16.

[Kluge]

Thank you very much. If a motherboard has two PCI-e slots (even if x16, x4), are they required to permit any 2 cards, even if they use the full form factor?

For example, if a motherboard manu. says their board can use two PCI-e 3.0 cards, does that necessarily mean I can fit any two cards which claim to be within the PCI-e 3.0 specs (assuming they have stock coolers)?


Edit: After reading many mobo reviews, it seems all mobos support all graphics cards, but some only just barely permit the card to fit and will have the cards touching each other.

[Litt]

I just saw someone selling that exact same board with sempron 140/1gb ddr3 ram/40gb HD for real cheap here on market place section here.

http://bitcointalk.org/index.php?topic=7528.0

Looks like he just reduced the price even more. This is a steal imo if you were looking to get these just for a 24/7 mining rig. Just add psu and gpus. He's selling a nice psu as well by seasonic.

[keybaud]

Some people use multiple HD5970s connected via PCE-e x1 slots. The PCI-e bus bandwidth limit isn't really relevant at all.