Depends on what you are doing with the PC, but you can definitely do better than an RX 580 in an 800 USD build:
PCPartPicker Part List
CPU: AMD Ryzen 5 3600 3.6 GHz 6-Core Processor ($174.99 @ B&H) Motherboard: MSI B450 Gaming Plus MAX ATX AM4 Motherboard ($104.97 @ Amazon) Memory: *G.Skill Ripjaws V 16 GB (2 x 8 GB) DDR4-3600 Memory ($72.99 @ Newegg) Storage: ADATA XPG SX8200 Pro 512 GB M.2-2280 NVME Solid State Drive ($84.99 @ Amazon) Video Card: EVGA GeForce GTX 1660 SUPER 6 GB SC ULTRA GAMING Video Card ($239.99 @ Walmart) Case: Fractal Design Focus G ATX Mid Tower Case ($62.87 @ Newegg) Power Supply: Corsair CX (2017) 550 W 80+ Bronze Certified ATX Power Supply ($68.98 @ Amazon) Total: $809.78 Prices include shipping, taxes, and discounts when available *Lowest price parts chosen from parametric criteria Generated by PCPartPicker 2020-03-02 15:54 EST-0500
The 1660 Super is about 30% better than an RX 580 and can still fit into an 800 USD (-ish ) build.
It would have been better suited in Graphics Cards or New Builds and Planning, but if the moderators feel like it has a better spot there, they will move your thread.
But please post some more context to know what you're dealing with.