Ok, so I've been looking at a lot of info, spec, etc with the new cards being out, and I've got a few questions about how some of the crossfire might work. First off, How would crossfire work with say a PCI-3.0 and a PCI-2.0? Would that just run at the PCI-2.0 "crossfire speed", or what? I read somewhere that the new crossfire system works fine on PCI-2.0, but there is a slight dip in performance compared to PCI-3.0. Would having the PCI-3.0 slots at different bandwidths (ie. x16 and x8) result in a slower "crossfire speed"? Also, I've yet to see any reviews of these in anything higher than 2 way crossfire, so are they 3/4 way crossfire compatible? I seem to remember hearing that they were, I would expect them to be at least.
Generation 2.0 does become a little saturated in crossfire configurations. It should still be worthwhile.
I know that PCIe gen 3.0 has a 1% performance boost with dual 7970s. So the bottleneck will be a little bit more with dual 290x
Is it true that z87 only runs PCIe 3.0 8x/8x for cross fire? edit: looks like it's true If that is true, it should be roughly the same as PCIe 2.0 16x/16x.
You're right. It depends if the motherboard supports x16 x16
So would CF 290x on a PCIE 3.0 x8 bottleneck?
Nope.
2.0 x16 x16 and 3.0 x8 x8 will be fine. Ensure that the motherboards run with those lanes.
Its more like pcie 3 runs actually the same as pcie 2 but every once inwhile the stars align and you get a few extra frames per second out of the pcie 3 . edit tho with the band width on the 290x it may mean more .
This all seems pretty reasonable for the most part. As for 3 and 4 way crossfire the only info I've found is a ln2 OC that someone did with a 4 way set up. It was on the Asus ROG page, so it would seem that there is still benefit in 4 way!
Various motherboards have different generations of PCIe, and different numbers of lanes. I wouldn't try to do 4 way on anything besides socket 2011. Sure, you can do 4 way on lower motherboards, but you're not going to get anywhere near 360% scalability. The performance value of 4 cards on a low bandwidth board will equal less than 3 cards on a high bandwidth motherboard.
The R9 290x use's crossfire over pcie only with no bridge now the older cards over the low power cards needed a bridge because there was not enuff bandwith to just use pcie gen 2 ...older crossfire dident need a bridge so i will be interested how that card handles over just pcie 2 the card is ment to use pcie gen 3 for crossfire without a crossfire bridge.
pc perspective have promised to test these cards over pcie gen 2 soon (thay only did them in gen 3)