I'm guessing your GF4 MX does not have another monitor output.
If you have Windows XP, it is easy. You will need another graphics card. If you can find an old PCI Video Card, it's all gravy. Provided it is not too old, and the drivers do not work.
Plug in your primary monitor into your AGP car (your GF4 MX), and the other monitor into the other PCI card.
Boot up as normal.
On your desktop, right click ->Properties->Settings
You should see two "Boxes" there with the one on the right grayed out.
Click on the grayed out box. Either right click and select "Attached" or in the lower left hand area, check off the option "Extend my Windows Desktop onto This Monitor".
Adjust your resolutions, enjoy.
Otherwise, it's always a good excuse to get the latest videocard with dual monitor output.
If you want a 2D only onw, I highly recommend the Matrox P650. Passively cooled (no noisy fan), has dual DVI outputs(comes with DVI->VGA adapter), and supports independant resolutions and refresh rates in Windows 2000.
-Ed