There are a few components i can't decide :
Processor : i5 4670 or i7 4770K
SSD : 120GB (for OS n Game data)
VGA : R9 270x vs R9 280x (Sapphire or Gigabyte?)
Cooling fan : that can fit into Full Tower Casing Corsair 750D or Cooler Master HAF-X NVDIA
Casing : Corsair 750D or Cooler Master HAF-X NVIDIA
I can afford for both of the options (VGA and Processor), but what i'm after is efficiency.
More like "Ok, for the next 3 years, i5 is more worth that having for now i7, bcoz..,.", or "For now R9 270x is more worth to buy than R9 280x bcoz...."
If you're targetting gaming then the GPU is way more important. It's really rare for modern processors to stretch their legs in gaming, maybe when you're powering 4 titans then yea, you might wanna consider a very powerfull cpu to avoid bottlenecks. But with only one or even two GPU's, that i5 will serve you perfectly fine. You won't see nearly as much performance increase from a more powerful CPU as from a GPU.
So my recommendation:
Take the i5, it will perform the same as the i7 98% of the time, and pair it with that 280x or even a 290 like a1blaster says. a 290 would rock...
preferably a Gigabyte card, they have been known for their quality and customer service and their cards seem to be a little bit better than Sapphire's
This is a link to a review of seven different 280x non reference models. It has all the info you need to make the right choice. temps, noise, benchmarks,...
As you can see the HiS performs pretty awesome in all the test. Notice the low noise and temperatures compared to most of the others. So not a bad choice if you ask me.
I'd check the whole article out and make a decision once you know everything there is to know about all those cards, the photos may even help you decide as some are prettier than others . then take a look at the price of each one and make a final decision.