for Geforce cards, what does "Use XX MB of sys memory for tex in PCI mode?

[Exiler]

for Geforce cards, what does "Use XX MB of sys memory for tex in PCI mode?

I dont understand what does this means?

Under the Nvidia Additional Properties, for BOTH Direct3D and OpenGL, there is an option where you can choose X number of system memory to be used for textures in PCI Mode. What does that mean?

And should the higher the MB, the better?

This is for Nvidia 6.31 Det driver for my Geforce 2 MX AGP.