An S-Video cable carries a few video signal components, but NO audio. You need to run also the stereo audio cable from the computer's stereo audio output jack to the TV's two RCA audio inputs - White is Left, Red Right. Set your computer's audio output to 2-channel stereo, not something like 5.1 multi-channel. Or you MIGHT get away with leaving your computer sound system untouched and plugging the audio cable into the Headphone jack on the front of you computer's front speaker pair.
In terms of signal quality (and hence picture quality on the TV), Composite Video is better than a modulated TV channel, and S-Video is somewhat better than that. However, neither is close to what a computer screen resolution is, nor HD TV with HDMI signals, etc. The ROUGH relationship of TV signals to computer screen common resolutions is like:
Regular TV (strong clean signal) 640 x 480 - 480i in HDTV terms
Composite Video a bit better
S-Video maybe 960 x 720, maybe not - that's 720i
Most TV's do not have ANY modes that end in "p" in HDTV terms, although HDTV's do have 720p or 1080p for Component Video signals from a DVD player.
No regular TV's have fast frame rates.
The standard TV signal consists of 525 "lines" of analog data per screen, although only about 475 to 480 of those are actually displayed on the screen; the remainder are used these to transmit hidden digital data. There is no fixed number of dots in one horizontal line of the analog signal, so comparison to computer screen resolution numbers is rough. A TV frame rate is 30 frames per second, always interlaced. That is, a full frame is transmitted as two half frames - the first half contains every second line sent in 1/60th of a second, and the second half frame contains the intervening lines (in another 60th of a second) to fill in. By comparison, most computer displays are sent as progressive scan signals - every line in one frame is sent in order - at 60 to 70 frames per second (double the TV rate), and high-res displays get over 100 frames per second. Moreover, the resolution in both directions is much higher (as I said, common TV signal is similar to 640 x 480i x 30 fps). The very best HD TV systems now marketed amount to 1920 x 1080p x 120 fps.
No TV can do much better than those performance numbers, so no video card will try to push higher-resolution data or higher frame rates out to a TV that can't use it. By the way, 25-foot signal cables can have the effect of reducing the signal quality (and hence detail) at the TV, but you may find the impact small enough to ignore.
Can you use three displays? Not likely. Check the docs on your computer's video card for details. I've seen commonly that a card can output 2 signals at once, such as a DVI to the monitor and a S-Video out to a TV, or a DVI and a VGA to two monitors. If you already have two 19" monitors running, my guess is you'll have to change the video card's output settings to get an S-Video signal to the TV and lose one monitor display. Of course, this can be changed again.
