OK, then does this
The guy sent a message to the Mandrake cooker list complaining of the same error as you're getting; it turned out to be the AGP driver he was using (well, a combination of that and which AGP mode his motherboard was able to do). Does using agpgart rather than NVAGP help? Or vice versa, using nVidia's AGP rather than agpgart?
If not, the contents of your X log would be helpful.