I am sharing this because I just did mine today -- I bought a graphic card off ebay to use for my dual monitor with windows 10 but it didn't work. Here is what I did to get it working.
1. go to this link and download the edition of the windows you are using -- for me I am using 64bit windows 10 pro -- so I downloaded Windows Vista - 64-Bit Edition --- https://www.amd.com/en/support/graphics/legacy-graphics/ati-radeon-x-series/ati-radeon-x1xxx-series

2. Install the downloaded file (just proceed with installation if you get compatibility error check)

3. go into device manager (for windows 10 just search for device manager in the textbox on the bottom left of your screen)

4. once you are in device manager you will see a list of hardware in your PC -- in my case I saw (Display Card) option apart from the basic micrsoft display adaptor

5. I right clicked on the display card option for my graphic card and selected the option to update driver

6.Choose "Browse My computer for Drivers"

7.Select C:/ATI (Please Note:If you have other letter,it doesn't change nothing,just select ATI Folder on your HDD Root)

8.Wait while it install Drivers -- if you already have your dual monitor connected like mine then you should see your monitor split and expand correctly

9. As I understand, this should work for Windows 8/8.1/10 -- It definitely work for my windows 10 pro

You can also rely on the link below to guide you with pictures
https://forum.xda-developers.com/showthread.php?t=1536711