cat /proc/driver/nvidia/agp/status tells you, if AGP is working
glxinfo tells you, if Opengl is working.
glxgears is a litte programm, ideal to test opengl.
less /var/log/XF86.0.log will show you X error messages.
dmesg will show you kernel messages.
The warnings about tainting are normal. The nvidia modul is binary only and the kernel tells that the user.