Return the 420mx and get a better one. It sucks anyways even if you get it working.
Get at least Gf4 4200 or stick with the old card unless it's a TNT2.
Obviously if the card's connector and monitors connector are different size, you'll need either a new monitor or an adapter. Maybe the gf4 has only a DVI out in which case you'll need an analog vga adapter to get out any signal from it.