Budget & Desired/Intended Use?
*Preferences? (New/Used, any particular Brand, Air/Liquid Cooled, Custom-Chassis, Etc.)
...
Did you just want a standard terminal (ATX/ITX)?
You could perhaps build a compact NUC?


*EDIT* -R.O.G.Build-
For roughly less than $1,200.00:
• ASUS ROG Strix X570-I (MoBo)
• AMD RYZEN 7 3800X 8-Core 3.9 GHz (Socket-AM4, CPU)
• CoolerMaster MasterLiquid ML240L RGB V2 (Closed-Loop, CPU-Cooler)
• 32GB G.SKILL Aegis [2x16] DDR4 3200 [F4-3200C16D-32GIS] (RAM)
• Sabrent 512GB Rocket M.2 (2280) SSD (C:/)
• 8GB ASUS AMD Radeon RX 5500 XT [RX5500XT-O8G-EVO] (GPU)
• ASUS ROG Strix ROG-STRIX-550G 550W (PSU)
+ Chassis

But honestly, I prefer EVGA -- you may want to consider looking into the new LGA-1200 Socket and 3××× Chipsets, later this year(*) though.
*Best of Luck!

AMD is cheaper but you have to be careful of the lower end cpu's as some of the motherboard bios may not support them.
Reuse the old keyboard and mouse and maybe the case (I would buy a new one as your old case may not support USB 3.0 - 3.1 or the new motherboard).
Buy new power supply as some of the old ones do not supply power to the video cards.
You can get away with 8 GB ram but I would go with 16 GB (kinda depends what you are using the computer for).
Monitor and video card are extra $$$ unless you use the old one.
Expect to spend $600 - $1000 on new build without new monitor and video card.