OK.....first if its just for gaming and you're overclocking, go with a 7950 instead of the 660Ti, as you will get much higher FPS and have no need for the CUDA cores in the NVIDIA card.
And yes, that Corsair TX750M is easly enough for that system fully over clocked......but regards the future upgrade plan, 16gb ram will be over kill for just gaming, so save that money and put it towards the second GPU.
Then I have another question. What is the best manufacturer for making HD 7950? I can choose from ASUS, Sapphire, Gigabyte, MSI and Powercolor. And why it's better than others?
First, It comes down to who offers the output you need......if you just running one monitor its not a issue as they all tend to have DVI, mDP and HDMIDisplay Outputs.
So then it come down to cooling, and I would tend to avoid the reference card design with just one fan, and look at some of the models with two or 3 fans as this will help keep the temps and noise down when you overclock.
If i was picking right now, id go with this Gigabyte 7950 Windforce: