I have the Noctua NH-D14 running my 2500k at 4.83GHz and it has been flawless for over two years since the build (55C load in summer ambient indoor temps of up to 80F). You will see an improvement with the Corsair H100i, but some have reported less cooling performance who are using the H100i over their D14. About the only thing you'll need to be aware of is how big that cooler is. Specifically, can it fit in your case and check your RAM slots and the height of the memory you are using. Those with really high heat sinks will not fit. My G.Skill Ripjaws however work fine with the ASUS P8P67 Pro. Also keep in mind this air cooler is very heavy and some people are uncomfortable putting that weight on a motherboard long term, especially if they move the PC around a lot (I do not). If I had the decision again, I'd probably go with the H100i. But I'm still happy with the air.
Now regarding the mobo, I'm not familiar with ASRock but have read good reviews from them both personal and technical. Your overclocking success will depend more on how good your chip is over your motherboard, so long as the mobo has the extended/detailed overclocking options, or good software for overclocking if you aren't really good at tinkering with every o/c setting manually.
The Sandy-E's are more testy in overclocking than the older original Sandys like mine, especially once you push it past 4.5GHz or so. Further, with the GTX 680, I really can't tell much difference in frames in most shooter games between 4.8GHz and 4.2GHz as they are more GPU dependent in general (Crysis 2, BF3, Far Cry 3). However, in some older games like Microsoft Flight Simulator X and the original Crysis, it makes a measurable difference. Just something else to think about. In any event, good luck!