My PC is an ASUS Prime B450 mobo with Ryzen5 2400G. two Samsung UHD monitors used 95% of the time as Win10 extended desktop. I do little gaming, none of it using both monitors. The PC's video ports (HDMI, DVI-D, VGA) can't drive both monitors at top resolution of 3480 x 2160, so I'm looking at GPUs with 2 HDMI ports, ideally under $200 (again, no high-end gaming needs). So far the 4GB Radeon Pro WX 3100 and 4GB MSI RX 580 Armor look like the best bets.
1) Win10 currently recognizes both the monitor connected via HDMI and the one connected via a DVI-D -> HDMI adapter cable, although the latter maxes at 1920 x 1080. Am I correct in assuming that Win10 will recognize two monitors on the two HDMI ports of a new GPU card and extend the desktop? 2) Am I correct in assuming that 4GB is enough for my purposes -- mostly office apps, occasional single-screen 1080p video and games? 2) Does a new GPU automatically take over from the Ryzen integrated graphics once the latter's outputs aren't connected, or are changes in BIOS setup needed?
If you decide for a workstation GPU, I suggest WX3200 since WX3100 will be EOL'ed soon. 4GB GPU should be enough. Depending on the motherboard, you might need to change the setting in the BIOS. Usually when an external GPU is installed the onboard GPU gets deactivated unless your primary choice in BIOS setting is set to Onboard.