Your HP EliteDesk 800 G5 SFF, as per: http://h10032.www1.hp.com/ctg/Manual/c06443940.pdf, is fitted with a 250-watt power supply with HP p/n: L28758-001.
These two graphics cards -to the best of my knowledge, should fit in your desktop (please verify dimensions) and can be powered with your stock power supply. These cards are powered solely through the PCIe x16 slot on which they are fitted:
Gigabyte Radeon RX 6400 D6 Low Profile 4GB GDDR6, such as this purchase option: https://www.amazon.com/GIGABYTE-Radeon-Profile-Graphics-GV-R64D6-4GL/dp/B09YVPZ3RY/ref=sr_1_97?crid=…, or -if you are willing to spend a little more, an HP Nvidia RTX A2000 6GB GDDR6, with this purchase option: Amazon.com: Computer Upgrade King NVIDIA RTX A2000 6GB GDDR6 (~3050) PCIe Professional Gaming Graphi…, or: https://www.ebay.com/itm/125666192375?epid=23051537244&hash=item1d4249ebf7%3Ag%3AE0AAAOSwlbljm7ec&am….
The latter graphics card option is by far the most capable and will give you the most bang for your buck, as shown here: https://gpu.userbenchmark.com/Compare/Nvidia-RTX-A2000-vs-AMD-RX-6400/m1611573vsm1834749.
Hope this was helpful.
Kind Regards,
NonSequitur777