As PSU goes yes I might ho lower, I want to do later SLI another 980TI but that has time.
Yeah, just saying even if you do a 750W unit will be fine. It's not that buying a 1000W unit will harm anything, it's just spending more than you really need to.
Cases considering i really wanted Cosmos 2 because it just looks really cool!:) but Looks like i will buy 900D in the end:)still not 100% sure thou.
If you're considering air cooling, the Coolermaster will likely do a better job - will expand on this below.
I did post on forums on overclockers but no one responded there:(
Was it in the main forums, or on the customer service forums? Could you PM me a link?
I also want to ask how big difference is there for Air cooling and water-cooling?
Essentially with water cooling you'll have more flexibility, e.g. you could have a very quiet watercooled setup that cooled as well as a noisy air cooler, or a noisy watercooled setup that cools better than any air cooled setup, or anything in between - you're limited only by your space and your budget. That said, you're unlikely to gain much through watercooling in this instance, as you have the budget to buy an excellent air cooler (such as the NH-D15 you suggested) which will be both quiet and perform well, and the 980Ti won't be hugely loud either. I have never recommended a closed-loop cooler such as the Corsair Hydro series, or many of the prebuilt kits, simply because they lose one of the main advantages of water-cooling which is flexibility and expansion.
Taking into account what you've said, I'd advise either sticking with high-end air cooling, and choosing an appropriate case (the Cosmos II would be better for air cooling than the 900D), or going with proper water-cooling but perhaps getting someone to build it for you if you're unsure. It's not enormously difficult but you do run the risk of causing damage, I built my first few watercooled systems with colleagues before I started going it alone and felt much more comfortable this way.
Overclockers will build you a custom system or make changes to their range to suit you, and they do watercooled setups, so again it might be worth either posting on their customer support area or giving them a ring to discuss your options if you want to explore the idea of them building you a watercooled system.
Hope this helps give you some food for thought