What brand is the current PSU you have? That's the only potential issue I see, aside from the mATX motherboard; I've seen heat problems with quad-core CPUs on mATX boards, although these were older boards and CPUs, and I suspect you might probably be fine with newer components. Someone else can give more/better insight into this, I guess.
Also, why buy the HD 6870? An HD 78xx series card would be a better buy.
I would say dont water cool for now. that will save you a bit of money. just go with a hyper 212 evo until you get some extra spare money. go for a core i5 3570K. and just get a amd card for now because they recently cut prices, and drivers 12.8 match it up with nvidia cards. also you should use that saved money to get a samsung 830 ssd because they are fast and reliable. also go with a modular power supply, that is at least silver certified(bronze if you really have to.). and if you are gonna sli, go with a 850w, otherwise go 750 if you are not. also get a modular if your case does not have good cable management. corsair offer the most reliable psu in my opinion. good luck with your build.
Passmark is a synthetic benchmark, which is not always indicative of real-world performance; as I stated earlier, some of the information can be grossly erroneous.
Look for reviews on well-known sites like this one (Techspot), HardOCP, Guru of 3D, techpowerup!, Anandtech, bit-tech, HEXUS, LegitReviews and Tom's Hardware, among others, and check performance for yourself before deciding. Quite often, the games you'd want to play the most often can decide what card is better to buy for you.