Well I was hoping to upgrade to a Vega64, but it just is not meant to be right now. Money is looking a bit tight for awhile. I was looking at the RX 590 release and was wondering if that would be decent upgrade. The price looks nice and would like to move up to a 8GB card. Running a i5 4670k, 16GB DDR3, and a 34" 2560x1080 Ultrawide Monitor (LG). Any suggestions would be welcome.
I would suggest you to look for a second hand Vega56 or GTX1080.
The rest will not be such an upgrade to you, especially if you come from a R9 290X.
If you want to step up and deserve it for holding so long with a 5 years old gpu, the only good deal for you are these 2 cards.
Obviously get them second hand, i would not even bother to buy them new, you can find some cheap around.
This will allow you to keep you gpu longer as you did with the R9 290X and upgrading the cpu later will provide you the best perf/bucks.
Nividia is out. Monitor I have is freesync so staying with AMD. Define cheap for second hand? Cheap for me is at least 50% off msrp and I haven't seen that at all.
Right now, I wouldn't really consider the RX 590. At $279 it is a decent card, but you can currently get quite a few decently overclocked RX 580s for $80 cheaper. The card isn't much slower than the 590 and you save a big hit to the wallet.
The next logical step up would be a Vega 56, with a reference model at $339, about $60 more than the RX 590 but substantially faster. The nice thing about Vega, is it also supports the full DX12_1 feature set, while Polaris is limited to DX12_0, so there is a little extra "futureproofing" there.
All gpu are overpriced nowadays, so again anything with Vega56 performances for around 300$ is quite cheap and a good buy.
I also suggest you to get an update on gpu's and revamp your idea of "cheap" when speaking about the modern gpu market, either new or second hand.
So again my best suggestion would be to save the money needed to get a Vega56, anything else is a waste of money coming form a R9 290X!