Alvaretto,
Do you have the documentation for your motherboard? When you say you have an "on board adapter", does that mean the video connection is on your "back panel" or is it an AGP slot with the SiS 305 plugged into it? And is the new 3DForce4MX an AGP or PCI card?
Even if you're able to answer those questions, I don't think I'd have an answer for you.

I'm thinking that your motherboard either doesn't support the configuration you're trying to set up - or there is a dip switch somewhere on the motherboard which you need to set correctly to allow you to use that setup. You'll need to look at the motherboard documentation for that info. Most manufacturers have motherboard specs/documentation on their websites. Try looking there for the details on your motherboard's configuration.
Jeanie