Mike,
How old is the card? Is it a GV-800A or B or does it not have a letter in it?
With the GV-800 A or B cards you can have up to two installed in the same DVR. However, make sure your hardware is capable of supporting them. Having more than one card at the very least will require a faster processor, more RAM, and a bigger hard drive. You'll also double the recording channels from 16 to 32. We've had some customers order the additional card and then have problems such as the DVR freezing up or become extremely slow (almost non-responsive) because the hardware could not handle the reqirements of the additional card.
Here's a link to the GV-800B:
http://www.geovision.com.tw/english/Prod_GV800B.asp
Hope this helps!
RYAN