The S-video output on most PC video cards will re-scale basically anything you feed it to it's notion of TV 640x480i usually EVEN IF IT'S ALREADY TV COMPATIBLE. They are a great way to get "something" on a TV quickly. They are not good if you want any real control over the timing of the video signal or the best quality you can get out of a given TV. Your best bet is usually either to poke around with various 640x480p options for the input to the scaler and see if you can find something that it doesn't touch too badly or to just crank things up to whatever the scaler input maximum is, typically 1042x768, and live with it all being (at least) double scaled.
The "VGA to composite" and "VGA to s-video" and even "VGA to component" cables that you'll find for $5 are all passive cables that assume the source device is capable of handling generating the proper electrical output or input on that connector and just needs a cable adapter. They're intended for some laptops and projectors that actually handled YPbPr component or S-Video over their HD15 "VGA" connectors with special settings. They won't work unless you have hardware designed for them which you probably don't.
If you have a TV that supports S-Video as its best option and want to have low-level control over the graphics timing, your best bet is to find a non-scaling RGB to S-Video adapter and use any one of the normal "hacks" to get 15k scanrates out of a PC. The adapters vary but cheap ones are typically about $15 and seem to all be the same design from China with different names and based on the Rohm chip. I personally think the Analog Devices AD725 is a little bit better, but both work fine. The results are typically not bad, but you'll still be limited by S-Video's limitations. 240p IS possible over S-Video, and most TVs will happily accept it. 480p S-Video can be meaningfully generated electrically, but most TVs won't work with it (480p composite is NOT meaningfully possible electrically).