Thanks, I'll try there as well
I tried using compiz with and without --replace gconf - the results were the same as the ones I described above. The thing is that compiz 0.0.13 used to run while Xgl was running (and working really well!) before I updated my nvidia driver...
Please let me know if you have any other suggestions. Also, is there a way to "revert" to the driver I had before the update (i.e. the kernel modules that installed with my mandriva)?