I believe there's a driver available for the Trident Blade card that will correctly output to a medium res monitor.
Not sure of this part, but I'll throw it out anyway. There are only a handful of games that run on a medium res monitor, and I don't believe you'd be able to run the rest on it properly. Given the choice, I'd either find another cab, or figure on replacing the monitor with a TV or CGA monitor. You might check to see if the monitor has any use to the seller, they may be willing to remove it and lower the price on the cab without it.