You need a free Molex power connector, attach it to your video card.
Molex:
http://www.ag.ndsu.nodak.edu/agcomm/...ages/molex.jpg
This is the much smaller floppy power connector:
http://www.abctechinc.com/Tests/geek1003-4.jpg
If there´s no Molex connector left you need to buy an "Y" splitter cable:
http://modstore.com/images/molexysplit.gif
Which card did you buy? Is your power supply powerful enough?