The original card will have come with an ADC port and a DVI port. If the card has not been changed out, then you are good to get either type of LCD.
I'd recommend getting a DVI monitor, unless you see a super deal on a used ADC monitor. Getting an ADC monitor will force you to stick with ADC video cards, or require you to buy a $100 adapter to DVI at some point. Somewhere in the future, that ADC adapter is going to become hard to find.