The motherboard is mATX which is smaller than ATX. For you it shouldn't matter since you don't plan to CF. I also suggested it because of Z68, you may not use Quick Sync but always a sweet thing to have for the future. The board also is one of the cheaper boards that is Z68.
It'd be fine. If you don't reach 4GHZ, not the motherboard (though there is a minor contribution) most likely it is the CPU. Not all 2500Ks are made to the same grade. Some can get it up to 5ghz and some with the same cooling can't get it stable above 4.6ghz. The board is sufficient.