I have a Sapphire X550XTX (01:00.0 0300: 1002:5657) to use this card I have to edit shared-core/drm_pciids.txt adding 0x1002 0x5657 CHIP_RV380|RADEON_NEW_MEMMAP "ATI Radeon RV370 X550XTX" and compile drm. This pciid should inserted in radeon driver too (I have to force it using ChipId 0x5b63 in xorg.conf).
Pushed to drm (0744cb153aabd290fd4832288a530adeb5561e2c), mesa (80efe27560134510dce88a52729e5a3d93e8e275, a107ec8300c6df3dc6ee636ccfd469c820b00190), radeon (0c26806245381b925b73ff9f3b747e3dcf0ebd6f).
Actually, the "X550XTX" is an rv410 based card (i.e. a rebranded X700) and not an rv370. See http://ati.amd.com/developer/ATI_Device_IDs.txt
Although from the perspective of the driver, there's not much difference.