In fact, just logged in as root, set to 1152x864, 24bit (32 bit in above message was a typo) and re-run glxgears and got the following:
[root@sol001 root]# glxgears
11133 frames in 5.0 seconds = 2226.600 FPS
13710 frames in 5.0 seconds = 2742.000 FPS
13719 frames in 5.0 seconds = 2743.800 FPS
13715 frames in 5.0 seconds = 2743.000 FPS
Dunno where the FX5600 is position in relation to the old Ge-Force 3, what other settings are you guys running ? Aliasing etc ? I've got everything on driver defaults as installed. Perhaps its more that GLX doesnt push the cards to their limits so they are hitting a plateau of performance.