I always owned ATX motherboards but I got tired of the big case size so I was thinking going MATX or even ITX but if it impacts my overclocking potential I'll stay ATX if I have to. Do I loose anything going from ATX to a smaller size? I overclock a lot my CPU, Memory, and Videocard I don't SLI at all I had nothing but bad experiences with dualcards. But I would like at least a bit of expansion ability for a sound card or SSD card in the future.
As long as you get a board with an appropriate power phase config then it won't have a huge impact on your overclocking. That being said: that means you need mATX, not ITX. A lot of ITX boards simply don't have the room to put on enough VRMs.
The good mATX boards tend to be more expensive than their counterparts. Because my case is under my desk I honestly don't care how big it is. I'd rather have plenty of room for anything than have a small "pretty" case.