Hi, Steve0916 -
To add to Robbie's good advice, unless the machine was special ordered with a dual-head card (one designed to feed separate signals to two monitors), the card it came with is designed to connect to a single monitor.
The fact that the card has two connectors does not indicate it is dual-monitor capable - often two different kinds of connecters are provided to increase the flexibility in connecting a single monitor.
It is not a good idea to connect two monitors to a single-monitor card. When two monitors are connected to a single-monitor card that way, if you're lucky you'll get a mirrored image on both monitors; if you're not lucky, you won't get any image (worst case is the card fries).
As far as the OS's capability goes, OS 9 has built-in multi-monitor support. FWIW, all Mac OS's since OS 4 or 5 or so have had that capability. My G4 Digital Audio machine has 4 PCI slots in addition to the AGP slot dedicated to a video card. If I added dual-head cards to all slots, in theory I could have 10 monitors connected, all in extended desktop mode - and the arrangement of them would be governed by settings in OS 9's Montors control panel.
However, as Robbie pointed out, card manufacturers may have a minimum OS requirement in order to use their cards. You'd have to check each one that you're interested in.
