What GPU to upgrade to?

Solution
I'd probably go AMD since you have a FreeSync monitor. Since I'm unsure on your power supply I'd also recommend upgrading it.

There is the RX 590 that recently released. But RX Vega 56 or 64 would be better, not sure by how much though.

Are you in the UK? or US? or elsewhere?
PCPartPicker part list / Price breakdown by merchant

Video Card: Sapphire - Radeon RX VEGA 64 8 GB NITRO+ Video Card (£488.21 @ Amazon UK)
Power Supply: Corsair - TXM Gold 650 W 80+ Gold Certified Semi-Modular ATX Power Supply (£76.44 @ Aria PC)
Total: £564.65
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-11-15 20:21 GMT+0000

PCPartPicker part...


CPU i5-6600k
Motherboard - MSI Z270 Gaming Plus
RAM - Corsair 16gbx1 3000Mhz
PSU - Cronus Modular 550W
Monitor 1920x1080 144hz Freesync
 


Maximum 500, maybe 550, 650 pushing it. I'd say I can spend 500 on GPU and 70 maybe on elsewhere if really neccessary.
 
I'd probably go AMD since you have a FreeSync monitor. Since I'm unsure on your power supply I'd also recommend upgrading it.

There is the RX 590 that recently released. But RX Vega 56 or 64 would be better, not sure by how much though.

Are you in the UK? or US? or elsewhere?
PCPartPicker part list / Price breakdown by merchant

Video Card: Sapphire - Radeon RX VEGA 64 8 GB NITRO+ Video Card (£488.21 @ Amazon UK)
Power Supply: Corsair - TXM Gold 650 W 80+ Gold Certified Semi-Modular ATX Power Supply (£76.44 @ Aria PC)
Total: £564.65
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-11-15 20:21 GMT+0000

PCPartPicker part list / Price breakdown by merchant

Video Card: Gigabyte - Radeon RX VEGA 64 8 GB Video Card ($504.98 @ Newegg)
Power Supply: Corsair - TXM Gold 650 W 80+ Gold Certified Semi-Modular ATX Power Supply ($69.99 @ Amazon)
Total: $574.97
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-11-15 15:23 EST-0500
 
Solution


Thanks a lot for this, I will likely get these.