Depending on your BIOS, it could also be possible that the onboard video gets disabled as soon as another VGA card is in the system. If you have, or can get, documentation on the BIOS features/settings for your motherboard, read through it and search for info on BIOS settings for video function.
If you can't get your computer to work with the GeForce2 MX 100/200 card installed, this probably means you'll need to access the CMOS Setup utility, where you can change BIOS settings, and disable onboard video. Assuming your NVIDIA GeForce2 MX 100/200 card only supports one monitor, you'll have to get and install a second video card. Or, you could discard your NVIDIA GeForce2 MX 100/200 card and get a card that supports two monitors.
I believe your system has an AGP slot. So, if your NVIDIA GeForce2 MX 100/200 card is a PCI card, you can get an AGP card and install it along with the PCI card, and vice versa.
I run dual monitors with two video cards, one AGP and one PCI. Both cards are nVidia chipsets. The nice thing about both cards being nVidia is that they use the same driver.