Pour info, j'ai quasi la même carte que toi mais je n'ai pas de problèmes avec.
Sans rien modifier suite à une install de Fedora12 :
$ glxgears
2801 frames in 5.0 seconds = 560.009 FPS
3014 frames in 5.0 seconds = 602.732 FPS
2830 frames in 5.0 seconds = 565.794 FPS
$ lspci -v
01:00.0 VGA compatible controller: ATI Technologies Inc RV370 5B60 [Radeon X300 (PCIE)] (prog-if 00 [VGA controller])
Subsystem: PC Partner Limited Device 0450
Flags: bus master, fast devsel, latency 0, IRQ 25
Memory at e0000000 (32-bit, prefetchable) [size=256M]
I/O ports at 9000 [size=256]
Memory at f1000000 (32-bit, non-prefetchable) [size=64K]
[virtual] Expansion ROM at f0000000 [disabled] [size=128K]
Capabilities: <access denied>
Kernel driver in use: radeon
Kernel modules: radeon, radeonfb
01:00.1 Display controller: ATI Technologies Inc RV370 [Radeon X300SE]
Subsystem: PC Partner Limited Device 0451
Flags: bus master, fast devsel, latency 0
Memory at f1010000 (32-bit, non-prefetchable) [size=64K]
Capabilities: <access denied>
J'ai heureusement un peu plus de FPS que toi 😉