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PCIE bus speeds and crossfirex wonders

Last response: in Motherboards
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Hi,

I have a Gigabyte P55-UD3R but i think its pretty much common to all brands.

Heres my wonder:

The specs states that its crossfirex compliant and that the pci-e slots would run at x16,x4 or dual x8.
When two pcie cards are slotted in, is the bus speed automaticly set for dual x8 or i can choose to run one at x16 and the other at x4?

If i can run it @x16,x4 what would be best cards to buy, i guess it would be a High-End + a low-End...?
What determines which bus speed it needs at least? Would a low end, ati radeon hd 5670 per exemple, run at full speed in a x4 pcie slot?

If i need to run it @dual x8, what would be best cards to buy? Would a higher end card get trottled by the bus?

thanks,

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Motherboard Expert

Normally x16/x4 would be used for a main graphics card with a weaker graphics used as a dedicated PhysX card.

There are a few articles here at Toms about the x8/x8 crossfire performance. In general its a small performance loss compared to x16/x16 (like 5%) even with high end cards like the 5870.

If you are seriously thinking about weak graphics cards like 5670s you should forget it and go with a single stronger card in the x16 slot.
 
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