installing second montior (can I use card from old computer?)

Hi I'm hoping someone can answer a question for me. I'm trying to
install a second monitor (as a surprise for my husband for use with
Flight Sim). I bought the monitor, but I haven't bought a video card
yet.

I have an old (old as in bought the computer in 1997) computer that
would have a card--can I use that or am I better off just going out
and buying a new card?
The computer it will be used on is a new Dell. Is there a way to know
which kind of card to buy?

Thanks for any info!