Sua placa já está com suporte nativo do novo Xorg 7! Mas só com suporte 2D segundo a documentação.
Segundo o site do Xorg:
Description
radeon is a Xorg driver for ATI RADEON based video cards. It contains full support for 8, 15, 16 and 24 bit pixel depths, dual-head setup, flat panel, hardware 2D acceleration, hardware 3D acceleration (except R300 and R400 series cards), hardware cursor, XV extension, Xinerama extension.
Supported Hardware
The radeon driver supports PCI and AGP video cards based on the following ATI chips
R100
Radeon 7200
RV100
Radeon 7000(VE), M6
RS100
Radeon IGP320(M)
RV200
Radeon 7500, M7, FireGL 7800
RS200
Radeon IGP330(M)/IGP340(M)
RS250
Radeon Mobility 7000 IGP
R200
Radeon 8500, 9100, FireGL 8800/8700
RV250
Radeon 9000PRO/9000, M9
RS300
Radeon 9100 IGP
RS350
Radeon 9200 IGP
RS400
Radeon XPRESS 200/200M IGP
RV280
Radeon 9200PRO/9200/9200SE, M9+
R300
Radeon 9700PRO/9700/9500PRO/9500/9600TX, FireGL X1/Z1 (2D only)
R350
Radeon 9800PRO/9800SE/9800, FireGL X2 (2D only)
R360
Radeon 9800XT (2d only)
RV350
Radeon 9600PRO/9600SE/9600, M10/M11, FireGL T2 (2D only)
RV360
Radeon 9600XT (2d only)
RV370
Radeon X300, M22 (2d only)
RV380
Radeon X600, M24 (2d only)
RV410
Radeon X700, M26 PCIE (2d only)
R420
Radeon X800 AGP (2d only)
R423/R430
Radeon X800, M28 PCIE (2d only)
R480/R481
Radeon X850 PCIE/AGP (2d only)