Currently I have a phenom x4 975 at 3.8 Ghz with 16gb of ram @ 1600 Mhz I also have a Radeon HD 7850
Should I keep the graphics card and upgrade the to a new motherboard and an intel 4770k,
or should I get an amd R9 290 gpu and keep the motherboard and cpu?
Which is best for gaming purposes?
Well, in either situation you're going to have to make further upgrades. Also, for gaming purposes, I see no reason to go with an i7.
While the Phenom is still a really competitive CPU, it would bottleneck an R9 290.
Switching to a more appropriate i5 will not boost your in-game performance, since that is largely determined by the 7850. Perhaps a tiny gain in FPS, but nothing seriously tangible.
I would probably advocate going with an up-to-date platform to secure certain features. Grab an i5 if you wish to go with Intel. The 7850 is still quite a good card for 1080p gaming.
You'll get more fps in games with a better graphics card, that CPU is fine.
Personally I would go for upgrading to an i5 4670K with a new motherboard to go with it and then add a R9 280x or a GTX770
What is your current motherboard? Maybe if you have a decent one, upgrading to a FX8320 with a R9-280X will please you more.
Grtz Angel ☺