As bc says, it depends. You didn't say how much you will use the finished basement. In my neighborhood, where building an addition will put you in your neighbor's yard, finished basements are used all the time. Hence, the main heating system is used to heat that room like the rest of the house. THEN, there are finished basements where, like a good friend of mine's home, the finished basement is only used for parties and gatherings. (It keeps everyone out of the upstairs which is REALLY nice.) Hence, their basement is part-time. What he did was put in electric heat and only turned it on when it was needed. Sure, it costs more to heat with electric but he insulated the walls and floor and he only uses it 10-20 times a year. That's a good solution in my mind.
If you plan on using your basment like you do any other room in the house (all the time), you can consider using your current system to heat it assuming it's properly sized. On the other hand, if the finished basement is used part-time, consider a completely separate source for heat or zoned heat with your current system. Basements tend to work differently than the rest of the house. What's equally important is to insulate the floor and the walls and address potential moisture issues. The only drawback to using a separate source for heat (or zoned heating) is that you'll have to wait a little while for the room to warm up. So you'll have to turn on the heat in advance of the people using the room. (This is another reason to insulate well.) Depending on your situation, consider a natural gas stove, zone heating, propane stove, or electric.