First, AMD does support the HD 7800 series GPU card with latest Windows 10 drivers from here: https://www.amd.com/en/support/graphics/amd-radeon-hd/amd-radeon-hd-7000-series/amd-radeon-hd-7850
If you want a less expensive newer AMD GPU Card it depends on your PSU wattage. You can use this website as a reference to what GPU card you can run on your computer depending on the PSU wattage you have installed from here: PSU REQUIREMENTS - RealHardTechX
How much money are you planning on spending and what will you be using it mainly for - Gaming, browsing, Rendering, etc.
The RX 500 series are still supported by AMD with the latest drivers. RX 570 and lower models cost from $170.00 or less at Amazon: https://www.amazon.com/s?k=Radeon+gpu&ref=nb_sb_noss_2