Crossfire with pci express slots at x16 and x4

The issue:

 Many motherboards with the mainstream intel chipsets(p55,p67,z68) were crippled by their manufacturers by having their two pci express slots 2.0 limited at x16 and x4 respectively.This decreased the second slot's bandwidth severely while offering minimal benefit from the first slot's extra bandwidth.These  motherboards were also labeled as crossfire compatible and were sold at lower prices than their more expensive counterparts which sported crossfire and sli at x8 x8 speeds.

  I was one of the people who bought one of these motherboards,the p7p55d, thinking that I wouldn't ever need the crossfire function.After a year or so however,I found a second Radeon HD5850 at a really good price and I was tempted to get it. At that time,I couldn't find any written proof that a setup like this would work without issues and I didn't want to risk throwing 200$ out of the window,so I skipped on the potential opportunity.

  For everyone with the same question today though, Tom's hardware did an informative article on this situation and published some interesting results.Let's take a look at them,shall we?

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Let me translate this into percentages:

 Allien vs Predator without AA showed a 10% drop going from x8/x8 to x16/x4

Allien vs Predator with 4xAA showed a 7% drop 

Crysis no AA showed a 0.5% drop

Crysis 4xAA  showed a 0,6% drop

F1 no AA showed a whopping 25,5% drop

F1 4xAA showed another whopping 24% drop

JC2 showed a 12% drop

JC2 4xAA showed a 12% drop once more

Stalker showed a 3% drop

And Stalker 4xAA showed a 3% again

  What do we make out of these?A crossfire x16 x4 setup is not advisable.While most games showed a light drop in performance,F1 and JC2 are the newest of the bunch.This indicates that there are games which require the bandwidth of the x8 slots and as game graphics develop this will only increase.

What if we use slower cards that require less bandwidth in sli and crossfire?Does Sli manage bandwidth differently and does that management change the performance?What is the difference in performance between an intergrated cpu pci-e controller and an external southbridge controller?So many questions without an answer...

  Then there's this article from Hardcop claiming that there is no visible performance difference between x16/ x16 and x4 /x4,except minimum fps . 

Bottom Line

 There is still no clear indication of how a crossfire/sli setup will work at x16/x4.My suggestion is that you get the best single card available and avoid the hassle of finding out yourselves and being potentially disappointed.This way you will also avoid the problems of multi gpu setups such as microstuttering and  bad compatibility with new and legacy games.

Sources:Tom's Hardware

HardOCP

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