Hi Jemain :
your problem is : You are not using direct rendering.
try this : glxinfo
you will see something like "direct rendrering : no"
So try to install the last nvdia drivers from their web site.
and after retry glxinfo and you will see something like
"direct rendering : yes"
AMD Athlon 64 3500+
Asus 6600 GT pciX
1024 Mo ddr 3200 Corsair