Getting RGB quality from PC to the TV

[Vertigo]

Sorry for the mess of this post and the myriad questions and typing while thinking (or not), but here goes.

I want: RGB quality picture from my PC that works on a non-prog-scan TV.
I don't want: To pay lots of money for a scan convertor.

I've got a GeForce 5600FX.
This card can do simultaneous output to both a monitor via standard VGA port and to TV (as a second monitor) via some weird SVideo-like connector.
My TV doesn't do SVideo, only RGB or composite.
What I want to do is take the ability of my graphics card to simultaneously output to these two displays but build a cable that combines the outputs to give me an RGB picture on the TV.
I'm thinking that I can do this in a similar way to how you get RGB from a NTSC Gamecube.
I think I can take the R, G and B signals from the VGA port, combine it with the sync signal from the TV output connector and get a clear picture, right?
It should be ok if I just set my VGA resolution to 640x480 shouldn't it? I should get a clear (albeit slightly bordered) picture, as my TV supports RGB at 60Hz in a PAL resolution (576 visible lines).
How do I get the signal for making the picture interlaced? My TV doesn't support progressive scan, which would make things much easier? Obviously this must be done in the TV output side of the card, but I'm not sure where it comes from.
If I combine the colour information off of the VGA side, but the sync and interlace information from the TV out side, this won't work, will it, unless I can make the card output exactly the same image to both 'monitors' at the same time. Is there an app that can do that, or is it generally possible with cards that support output to two screens anyway?
Does anyone have a solution to making RGB quality TV output video just by signal combination, or will I have to make this circuit instead? That circuit doesn't actually interlace the video, does it, because that's done as a feature of AdvanceMAME, isn't it?
Anyone have any ideas?

[NFG]

You're boned.  MOdern video cards treat the output as two displays, you can't get sync from one and video from the other, since they'll be different images/scan rates/etc.

I had an old card, a Riva128, that would do one or the other; if you booted into TV mode it didn't disable the VGA, so you could use the RGB from it.  New cards don't do it this way.

The only other way to do it is use a utility like Powerstrip that forces your video card to do resolutions Windows would rather it avoided.

[Guest]

Not even if I could get the card to output the same image to both screens simultaneously?

[Vertigo]

Bah, that's me, obv. And I had this to auto log-in too.

[NFG]

The problem is Windows won't let a VGA/DVI output port do a low-res screen, it flat out refuses, so the best you'll get without powerstrip is one 640x480 (or 800x600) non interlaced image, and a 640x480 interlaced image for the TV.  That simply won't work.