Your card is a GeForce3. Only GF2MX/GF4MX/GF4Ti cards can do dualhead display. With GeForce three, the two video output connectors are mutually exclusive, and are there mainly to allow you a choice of connection type based on whatever monitor you have (ie. use the VGA for most normal monitors or the DVI for better quality flat panels.).
Sorry to have to be the bearer of bad news.