hello guys i already had an 2500k cpu with a 6950 gpu but i dont know wich motherboard will be the best because in a future i will add another 6950. i heard that some motherboards have problems when you add the second gpu it will run x8 instead of x16
That isn't a problem -- it's normal behavior. With a second GPU, most P67/Z68 boards will switch to x8/x8 operation.
If you have a single monitor, then x8/x8 CrossFire performs the same as x16/x16 CrossFire. It's only when huge multi-monitor resolutions (like 5760x1200 or 5760x2560) are used that x16/x16 begins to pull ahead in performance.
Leaps from Shadows gives you a couple great boards that will all do the job you are looking for. With the Intel Core i5-2500K before you start to overclock it make sure that you add a good 3rd party HSF. Good luck and enjoy the new system.
On this page for example we see conflicting results....with AA off, we see x8 x8 outperforming x16 x16 .... but turn AA on (who wouldn't) and the x16 x16 outperforms 99 to 91 ....almost a 9% improvement ... on other games the lead was consistent but the amount was smaller.
It's not a "problem", just a bandwidth limitation. For the most part, there is a small performance difference .... tho I'd be most anxious to see the impact on minimum (rather than average) frame rates as well where I think the impact would be more pronounced. Still for $30 more than the next MoBo down w/o the NF200, seems a "why the hell not" decision.