arkutakalin :
If I buy a new core i7 4790 with Gigabyte Z97 motherboard including Gigabyte G1 GTX 970, will it become a powerful pc?
I suppose it will become a beast pc for gaming and work
It will be one of the most powerful mainstream computers. The only way to go faster would be to get a larger socket. The chips you're considering are on the modern socket with 1150 pins (LGA1150), which is the smaller option from Intel. The fastest CPU on LGA 1150 is the 4790k, which has 4 hyper-threaded cores (i.e. Windows thinks there are 8 cores). The "k" means the multiplier is unlocked so you can manually turn up the CPU's speed higher than the standard 4.0 Ghz.
To get a more powerful CPU than that, your only option would be to get a CPU on the larger socket with 2011 pins (LGA2011-3). There are three non-Xeon chips (all i7 CPUs) that fit that chip: 5820k, 5930k, and 5960x. Each is more powerful than the 4790k in
most cases. They also allow you to run more things from the PCI-e slots (e.g. video cards, sound cards, RAID cards, TV-Tuner cards, whatever cards). The 4790k can run the least PCI-e cards, then the 5820k, and then the others run the most. In terms of cores, the 4790k has four, the 5820k and 5930k have six, and the 5960x has 8 (all have hyper threading, so Windows will think there are twice as many cores).
There's generally no need to spend the extra money on stepping up to the 2011-3 socket for people who just game and do non-professional rendering. The primary exception is for gamers who use more than 2 video cards in SLI. The 4790k allows 2 videos cards, but nothing else in the PCI-e sockets. The 5820k allows 3 video cards with another small add-on card, and the other CPUs allow 4 video cards with two additional add-on cards. There are other things you can stack in the PCI-e slots, but I'm talking about reasons gamers might buy the LGA2011-3 CPUs.
If you have no add-in cards besides graphics, just get the 4790k. It's fine and will allow two high-end GPUs. Plus it's generally better for gaming. When you add more cores to a CPU, they have to be slowed down so they don't overheat. That means that each core on the the 4790k is faster than any core you'll find on an LGA2011-3 CPU. People pay for extra cores only when their software knows how to use them, and games generally can't use more CPU cores (or threads from Hyper-Threading) than the 4790k has. I think it's the best CPU in existence for a gaming-only machine.