This is likely a public supported driver for your distro (if it exists) that gpu is no longer supported and only legacy drivers exist. I would as the question in your support forum for your distro. If none can be found you might look into another flavor of Unix? Ubuntu has pretty decent support from their community for the older AMD gpus. Not say debian does or does not. I just don't have much experience with that.
On Debian I think what you're looking for is the xserver-xorg-video-radeon package. It says this:
This package provides the 'radeon' driver for the AMD/ATI cards. The
following chips should be supported: R100, RV100, RS100, RV200, RS200,
RS250, R200, RV250, RV280, RS300, RS350, RS400/RS480, R300, R350, R360,
RV350, RV360, RV370, RV380, RV410, R420, R423/R430, R480/R481,
RV505/RV515/RV516/RV550, R520, RV530/RV560, RV570/R580,
RS600/RS690/RS740, R600, RV610/RV630, RV620/RV635, RV670, RS780/RS880,
RV710/RV730, RV740/RV770/RV790, CEDAR, REDWOOD, JUNIPER, CYPRESS,
HEMLOCK, PALM, SUMO/SUMO2, BARTS, TURKS, CAICOS, CAYMAN, ARUBA, TAHITI,
PITCAIRN, VERDE, OLAND, HAINAN, BONAIRE, KABINI, MULLINS, KAVERI, HAWAII.
More information about X.Org can be found at:
This package is built from the X.org xf86-video-ati driver module.