If the Radeon RX 570 and RX 580 cards are too much for your build, but the RX 550 is too little, perhaps the RX 560 is just right. AMD's refreshed Polaris chip is showing up aboard more cards like MSI's Radeon RX 560 Aero ITX OC. The card comes in flavors with 2GB or 4GB of GDDR5 memory running at 7 GT/sec.
Cards like this model will probably be among the fastest Radeons that don't require power adapters for a while. AMD actually calls the GPU atop these cards "Polaris 21", and it's worth noting that the chip doesn't appear to be just a rebadge of the Polaris 11 GPU present in RX 460s. All the CUs in the Polaris 21 chip are enabled, for a total of 1024 stream processors. On the MSI RX 560 Aero ITX OC on display, the GPU is clocked at 1196 MHz, although other cards with auxiliary power connectors will likely have higher clocks.
MSI equips the RX 560 Aero ITX cards with an HDMI port, a dual-link DVI-D port, and a DisplayPort connection. Given that the cards are just a hair over 6" long (or 15.5 cm), they should fit just about anywhere you've got two full-height expansion slots. The new offerings don't appear to be available just yet, though Newegg has the 4GB model listed for $115.