Correct.
Power Supply Units have an "efficiency curve". - That will of course change depending on the load (I.E. Gaming vs Desktop), temperature (I.E. Fans spin up using more energy, PSU efficiency goes down the hotter it gets.)

For a single Radeon RX 580 and a Ryzen 2700X however, you shouldn't need anymore than a quality 600w-700w unit. And please get a quality unit, it should outlive your PC that way.

I would invest in a quality motherboard anyway. - The DDR4 speed will impact CPU performance, which will impact turn times... So don't underestimate it, I would go with a smaller SSD (500GB~) if it meant faster Ram, but that's just me personally.

I think this will be my final update. Found a decenly priced monitor with Freesync. Changed the 580 t0 4GB VRAM to save a bit of money. Besides 8 GB VRAM is overkill for 1080p. 

How's my current power supply? Is EVGA reliable? Is Gold Standard quality enough? Or should I drop an extra $50 on Titantium/Platinum? 

If I were you, I'd go with the 8GB variant of the 580. Not just because the extra memory (not as overkill as you may think), but also because most 4GB boards have slightly lower clocks for the memory, so you're losing not only capacity, but also performance.

A Gold rated PSU is more than enough. The extra price of higher rated models aren't worth it unless your electricity is very expensive. If you want to spend more money on a PSU, do it by going with a fully modular unit.

I don't know why you would get a 4 GB graphics card over an 8 GB one. Do you want your build to be obsolete in a year? The RX 580 8GB were had for $349.99 during the mining craze (cheap) but you can probably find one for $299.99 or less now.