PC to Plasma TV. Two video cards or One with dual output?

I have a Gateway GT5453E computer with a Gateway ECS MCP61-P AM2
Mother Board (on board NVIDIA GeForce 6150SE nforce 430 128mb
Video. I am getting tired to having to disconnect the vga wire from
my monitor to then connect to the plasma. I would like my plasma to
be permanently connected via vga cable to my computer. I was hoping
to put in a used video card I had, as a second video card to to the
trick. But for some reason, when I tried to install two different
pci video cards, my computer would not recognize them. Are the older
video not compatible, if so I would buy a new one. Or is that not
the best option either. Would disabling the on board video card, and
buying a video card with dual output be the better option? You would
think having two video cards would work better since they both have
their own processors doing the work. I am not looking for state of
the art video card, just something that will do the job and not cost
too much. What would you recommend? Thanks