I just got a TV tuner card, and tried wiring my Saturn on my PC. The picture quality is quite crap compared to my TV though. But how about a AV converter (i.e the one at Lik-Sang)? Will the VGA be anywhere near TV-quality?
A friend of mine hooked his PS2 up to his tuner card... same problem. He got an S-video cable and it looked better, but mind you it probably looks bad on your TV as well if you go sit as close to it as you do to your monitor... moreover computer monitors have way higher resolution, so you can see the artifacts much better.
TV-Tuner/75-Ohm coax is the SHITTIEST video quality you can get. They are only made for very old televisions which do not have RCA inputs (very old as in made before 1986). Never use one of these unless you absultely have to. The reason it looks so bad is because it carries all video and audio data on one wire and results in everything picking up noise since the tuner has to filer it all back out to separate signals.
RCA cables are much nicer. It carries composite video on one cord and left/right audio on separate ones. However it is still mixing R, G, B, V-sync, Sync, and Ground on that video line.
S-Video uses luminence and chrominence. It's a totally different kind of video signal which was not around till recently. It's quite nice for the price if your TV supports it.
Straight RGB is the best way to go. Each line carries R, G, and B separatly with video sync. It's the cleanest there is next to VGA.
If you live in Europe, you may want to look into a SCART cable since that would be your cleanest picture (as I recall SCART is a DB-25 conection and carries all the signals on different pins).
If you want to run it to a capture device on your PC, it's going to look pretty bad regardless unless you have a direct RGB capture device (which I have never seen). Then you would have pixel-perfect 320x240 (or whatever res it is).
An RCA connection would SLIGHTLY improve the look of it on your comp, but if it would cost more than $10, it's really not worth your time. S-Video would be considerably better. Go that route if it's available to you. Otherwise just deal with how it is now.
Remember this: REAL scanlines (as in not that half-assed shitty scanline "emulaiton" some emulators claim to support) save video. Even RealVideo stuff looks better on a TV.