You mean you're using the two outputs on the graphics card, right? (They're both digital/analog.)
Anyway, what product you need depends on what you want to do:
1. Mirror the content of your analog display on a TV.
2. Create a 3rd display, containing more extended desktop.
3. Add an NTSC preview monitor for video editing.
For 2, you can add a PCI graphics card (read: Radeon 7000), which will have the necessary output.
For 1, you can either add the PCI graphics card, or you can use a scan converter, which takes VGA and makes NTSC out of it.
For 3, assuming you're working with DV video, you can use a FireWire digital video converter. If it's not DV, then whatever capture card you use should also have output.
tooki