If you can wait another few weeks for the new AMD 7970 to come out it would be better. Also you don't need the 2600k better go for 2500k and just overclock, plus the PSU is too much, go for something around 800W max. If you buy the cheaper CPU and PSU, and maybe take out the Network and Sound cards (the motherboard has a good ones) you can go for a SSD which would realy increase your general perfomance.
And really a self built system will last just as long or longer than a pre-built computer. An Alienware/HP and this PC arnt really that different if you think about it. They still get their parts from other companies, and all those parts are put together by a bunch of people being paid too little. Sometimes the company will use cheap PSUs and then the whole computer will die earlier. And since nothing uses shitty capacitators anymore there really isnt any reason to be afraid of your computer dying earlier than normal.
Good luck and lots of fun with your new build! It looks awesome!