hi guys, i want know which mobo stable and fast for overclock
Pro+Cooler: i5-2500k + hyper 212 plus
First of all -- I think you'll be thrilled either way you go. I personally bought a a P8P67 (plain-vanilla version) fairly recently and love it and don't regret not buying the z68 at all. Having said that, let's break down what differentiates the P67 and the z68:
1) Z68 has SSD Caching
2) Z68 offers access to HD3000 (in the case of the "k"-sku chips) graphics, allowing both:
-- integrated graphics to serve as a backup to your discrete card
-- support for accelerated video transcoding, which Intel calls quicksync
On caching -- there are several reports out there that indicate that the performance gain provided by the HD caching is minimal, some might say the gain is on the margins. I wouldn't pay extra for the HD caching.
On using HD3000 as a backup -- it's possible that your video card will fail at some point. It would indeed be helpful to have the onboard graphics just-in-case. Worth paying extra for? Doubtful; in a pinch, you could always run to fry's and snag some $20 no-name to use for the couple of days when you're RMA'ing. But we should note that video card failures are fairly rare.
On quicksync -- one note about quicksync is that it will only help at all with transcoding; it will not help in the common circumstance in which you rip a DVD or bul-ray. Quicksync offers a benefit only when decoding data; therefore, the benefit it offers is speeding-up the decoding of an already digitized file before transcoding it to another format or to a smaller resolution. This would be useful if you have a ton of high-res movies sitting on a HD right now and you want to be able to quickly download them to your iPhone or something like that.
On price -- if you look at newegg, you'll find that there are bargains to be had on z68. For example, there is not much delta between the price of ASRock's PRO3 P67 and Z68 boards. On the other hand, there is significant deviation between the price of the Extreme4 P67 and the Extreme4 z68. It's the same story with Asus's P8P67 and P8Z68.
If it were me and I wanted something with more features than the ASRock Pro3, I would -- and did -- go P67. But, if the Pro3 were enough, I'd probably go ahead and pay the extra nine bucks...
Good luck; like I said, whether you go with P67 or Z68 Sandy Bridge is really something else.