I also filed a more lengthy bug message to the xorg people because I'm unshure about the reasons behind this. See:

https://bugs.freedesktop.org/show_bug.cgi?id=10321

Though glxinfo shows that direct rendering is enabled, most expensive tasks nearly freeze the screen because CPU-load is at 100%. This is true for things like text rendering in the terminal but also for the compositing extension to xfwm. OpenGl apps don't show up anything in their windows. Below is the glxinfo result. For the  X-log please see the link above.

libGL warning: 3D driver claims to not support visual 0x4b
display: :0  screen: 0
direct rendering: Yes
server glx vendor string: SGI
server glx version string: 1.2
server glx extensions:
    GLX_ARB_multisample, GLX_EXT_import_context, GLX_EXT_texture_from_pixmap, 
    GLX_EXT_visual_info, GLX_EXT_visual_rating, GLX_MESA_copy_sub_buffer, 
    GLX_OML_swap_method, GLX_SGI_make_current_read, GLX_SGI_swap_control, 
    GLX_SGIS_multisample, GLX_SGIX_fbconfig, GLX_SGIX_visual_select_group
client glx vendor string: SGI
client glx version string: 1.4
client glx extensions:
    GLX_ARB_get_proc_address, GLX_ARB_multisample, GLX_EXT_import_context, 
    GLX_EXT_visual_info, GLX_EXT_visual_rating, GLX_MESA_allocate_memory, 
    GLX_MESA_copy_sub_buffer, GLX_MESA_swap_control, 
    GLX_MESA_swap_frame_usage, GLX_OML_swap_method, GLX_OML_sync_control, 
    GLX_SGI_make_current_read, GLX_SGI_swap_control, GLX_SGI_video_sync, 
    GLX_SGIS_multisample, GLX_SGIX_fbconfig, GLX_SGIX_pbuffer, 
    GLX_SGIX_visual_select_group, GLX_EXT_texture_from_pixmap
GLX version: 1.2
GLX extensions:
    GLX_ARB_get_proc_address, GLX_ARB_multisample, GLX_EXT_import_context, 
    GLX_EXT_visual_info, GLX_EXT_visual_rating, GLX_MESA_allocate_memory, 
    GLX_MESA_copy_sub_buffer, GLX_MESA_swap_control, 
    GLX_MESA_swap_frame_usage, GLX_OML_swap_method, GLX_SGI_make_current_read, 
    GLX_SGI_swap_control, GLX_SGI_video_sync, GLX_SGIS_multisample, 
    GLX_SGIX_fbconfig, GLX_SGIX_visual_select_group
OpenGL vendor string: Tungsten Graphics, Inc.
OpenGL renderer string: Mesa DRI R200 20060602 AGP 8x x86/MMX+/3DNow!+/SSE2 TCL
OpenGL version string: 1.3 Mesa 6.5.2
OpenGL extensions:
    GL_ARB_imaging, GL_ARB_multisample, GL_ARB_multitexture, 
    GL_ARB_point_parameters, GL_ARB_point_sprite, GL_ARB_texture_border_clamp, 
    GL_ARB_texture_compression, GL_ARB_texture_cube_map, 
    GL_ARB_texture_env_add, GL_ARB_texture_env_combine, 
    GL_ARB_texture_env_crossbar, GL_ARB_texture_env_dot3, 
    GL_ARB_texture_mirrored_repeat, GL_ARB_texture_rectangle, 
    GL_ARB_transpose_matrix, GL_ARB_vertex_buffer_object, 
    GL_ARB_vertex_program, GL_ARB_window_pos, GL_EXT_abgr, GL_EXT_bgra, 
    GL_EXT_blend_color, GL_EXT_blend_equation_separate, 
    GL_EXT_blend_func_separate, GL_EXT_blend_minmax, GL_EXT_blend_subtract, 
    GL_EXT_clip_volume_hint, GL_EXT_compiled_vertex_array, GL_EXT_convolution, 
    GL_EXT_copy_texture, GL_EXT_draw_range_elements, GL_EXT_fog_coord, 
    GL_EXT_histogram, GL_EXT_packed_pixels, GL_EXT_point_parameters, 
    GL_EXT_polygon_offset, GL_EXT_rescale_normal, GL_EXT_secondary_color, 
    GL_EXT_separate_specular_color, GL_EXT_stencil_wrap, GL_EXT_subtexture, 
    GL_EXT_texture, GL_EXT_texture3D, GL_EXT_texture_edge_clamp, 
    GL_EXT_texture_env_add, GL_EXT_texture_env_combine, 
    GL_EXT_texture_env_dot3, GL_EXT_texture_filter_anisotropic, 
    GL_EXT_texture_lod_bias, GL_EXT_texture_mirror_clamp, 
    GL_EXT_texture_object, GL_EXT_texture_rectangle, GL_EXT_vertex_array, 
    GL_APPLE_packed_pixels, GL_ATI_blend_equation_separate, 
    GL_ATI_texture_env_combine3, GL_ATI_texture_mirror_once, 
    GL_ATI_fragment_shader, GL_IBM_rasterpos_clip, 
    GL_IBM_texture_mirrored_repeat, GL_INGR_blend_func_separate, 
    GL_MESA_pack_invert, GL_MESA_ycbcr_texture, GL_MESA_window_pos, 
    GL_NV_blend_square, GL_NV_light_max_exponent, GL_NV_texture_rectangle, 
    GL_NV_texgen_reflection, GL_OES_read_format, GL_SGI_color_matrix, 
    GL_SGI_color_table, GL_SGIS_generate_mipmap, GL_SGIS_texture_border_clamp, 
    GL_SGIS_texture_edge_clamp, GL_SGIS_texture_lod

   visual  x  bf lv rg d st colorbuffer ax dp st accumbuffer  ms  cav
 id dep cl sp sz l  ci b ro  r  g  b  a bf th cl  r  g  b  a ns b eat
----------------------------------------------------------------------
0x23 24 tc  0 32  0 r  y  .  8  8  8  8  0 24  8  0  0  0  0  0 0 None
0x24 24 tc  0 32  0 r  y  .  8  8  8  8  0 24  0  0  0  0  0  0 0 None
0x25 24 tc  0 32  0 r  y  .  8  8  8  8  0 24  8 16 16 16 16  0 0 Slow
0x26 24 tc  0 32  0 r  y  .  8  8  8  8  0 24  0 16 16 16 16  0 0 Slow
0x27 24 tc  0 32  0 r  .  .  8  8  8  8  0 24  8  0  0  0  0  0 0 None
0x28 24 tc  0 32  0 r  .  .  8  8  8  8  0 24  0  0  0  0  0  0 0 None
0x29 24 tc  0 32  0 r  .  .  8  8  8  8  0 24  8 16 16 16 16  0 0 Slow
0x2a 24 tc  0 32  0 r  .  .  8  8  8  8  0 24  0 16 16 16 16  0 0 Slow
0x2b 24 dc  0 32  0 r  y  .  8  8  8  8  0 24  8  0  0  0  0  0 0 None
0x2c 24 dc  0 32  0 r  y  .  8  8  8  8  0 24  0  0  0  0  0  0 0 None
0x2d 24 dc  0 32  0 r  y  .  8  8  8  8  0 24  8 16 16 16 16  0 0 Slow
0x2e 24 dc  0 32  0 r  y  .  8  8  8  8  0 24  0 16 16 16 16  0 0 Slow
0x2f 24 dc  0 32  0 r  .  .  8  8  8  8  0 24  8  0  0  0  0  0 0 None
0x30 24 dc  0 32  0 r  .  .  8  8  8  8  0 24  0  0  0  0  0  0 0 None
0x31 24 dc  0 32  0 r  .  .  8  8  8  8  0 24  8 16 16 16 16  0 0 Slow
0x32 24 dc  0 32  0 r  .  .  8  8  8  8  0 24  0 16 16 16 16  0 0 Slow
0x4b 32 tc  0 32  0 r  .  .  8  8  8  8  0  0  0  0  0  0  0  0 0 Ncon