Have you downloaded and installed the linux video drivers from nvidia's website?
This is their Linux driver page
Follow the instructions to install the driver. The Nvidia driver that comes with Linux distros doesn't support 3-d accelaration, which is why your games run so slowly. You need the Nvidia proprietary driver to use all the features of your card. Sadly, Nvidia will not make their source code available.
I think with Mandrake that you will have to download the kernel source code - this should be available as an rpm package on Mandrake's website - and install it before installing the driver. I use Fedora myself, so if any Mandrake users read this, please help if you can!
I hope this is some help to you.