Thank to you and all others who have provided advises.I have reinstalled dicover and now DRI is working right fine : ppracer is playable again. I think I had a problem with the agpgart module even if it looks like it was loaded just fine.
Again thank to you !!! Regards, JFPS : there is still something stange : glxgears still give 1600 fps.... but I don't really care....