My sons gaming computer is a 7 year old AMD Quad Core Phenom II. It is overclocked to 4Ghz, and in it's day was a very fast processor. Technology has moved on, and it is now therefore roughly equivalent to a fast Haswell i3. It is still fine for gaming, but will probably need to be replaced soon
If you buy an i7-4790k, you will probably still be happily using it in 5-7 years time.
Heck, I expect my build to still be good in 5 years time....