What is 'comp' in your terminology? Component or Composite?
As for as I know, Sony WEGA doesn't have a Component plug in front Panel. Are you sure it's not AV IN? (Audio, Video In)
AV in has three pins(Yellow, Red, White/Black) while Component has three Pins with RED, GREEN & BLUE color. The yellow Pin in AV is called as composite.
Why don't you try the Sony's flagship S-Video cable? All Sony TV's having S-Video. (I hope old SONY models will accept only one Component which you have already used to connect your DVD Player, isn't it?)
Have you run the TV CONNECTION wizard from your NVidia control panel?
Follow these steps:
1. If you have 2 COMPONENT Input in your TV, connect your system thru component. (I hope you already used one for DVD)
2. If you have only one COMPONENT In, then use the S-VIDEO out of your 7900 to connect to your TV.
3. Run NVidia control panel TV Wizard, choose the correct connector type, then choose the format as 576i (I hope you are not using SONY HD TV), then select CLONE/DUAL mode.