Hello I have an HP Compaq Pro 6305SFF. Currently has AMD A4-5300B APU with Radon(tm) HD Graphics 3.40Ghz.
I have 24GB installed. not sure what is the best CPU and video card I can use on this platform.
The max cpu is the amd A10-5800B (not K). Because this is very hard to find, I recommend AMD a10 5700. As for gpu, any low-profile GPU can run on it (for example a LP RX550 or an Nvidia 1650)
Take a look at this page for your HP PC which lists all the Hardware it supports and features: https://support.hp.com/us-en/product/hp-compaq-pro-6305-small-form-factor-pc/5296265/document/c03522...
Important: Before you upgrade to a processor not listed above I would contact HP Support to make sure the BIOS version you have installed will support the new processor.
The AMD A10-5800B is a OEM Processor according to CPU World: https://www.cpu-world.com/CPUs/Bulldozer/AMD-A10-Series%20A10-5800B.html
These are the GPUs that are compatible with the A10-5800B:
NOTE: All the GPUs listed are not supported by AMD anymore with Driver updates.