TV Tuner questions

I received the ATI TV wonder elite tv tuner
http://www.ati.com/products/tvwonderelite/index.html
as a gift and had to alter the metal frame to fit in the large PCI bus slot. I believe that I removed a modem but wanted to make sure exactly what I have removed, and that it is not neccessary to keep. It is: Conexant Systems Model RD01-D270
The new PCI bus fit in fine and seems to be working except when I try to play a DVD there is no audio.
Also a ATI Radeon® 9600 or newer graphics card is recommended for optimal display output quality, how much of a difference would this make compared to what I now have? And will a graphics card fit in the other smaller pci slot?

My cpu is a:
Dimension 4600C (2yrs old)
Pentium 4
2.66GHz, 512Mb of ram
I believe the soundcard is SoundMax, but I don’t know how to check the graphics card.

Thanks for the help