Will this work with an XFX Radeon RX550 video card? I cannot find anything that says if their DisplayPort is ++ or not.

Works perfectly with my Aero ITX Radeon RX560, likely that it'd work for yours as well.

Questions & Answers: EnLabs PSDP2DVI DisplayPort to DVI DVI-D Adapter, Gold Plated 1080p DP to DVI Passive Converter Male to Female, Compatible with Lenovo, Dell, HP and Other Brand - 1920x1200

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EnLabs PSDP2DVI DisplayPort to DVI DVI-D Adapter, Gold Plated 1080p DP to DVI Passive Converter Male to Female, Compatible with Lenovo, Dell, HP and Other Brand - 1920x1200

Passive DisplayPort to DVI-D Converter
Connect a DisplayPort equipped computer to a DVI Monitor or Projector
Resolutions up to HDTV1080p & Computer 1920x1200
Requires a DisplayPort(DP++) Source

$9^.99 –