Best S Video Cable

Does your source equipment have RGB also?
You can also buy a switcher that will convert composite, s-video and componant, all with analog or digital audio to componant and analog/digital audio output.
You can check http://www.youdoitelectronics.com they carry switchers and such.
This sould take care any issues you might has now and in the near future to your existing monitor.

S video only will pass a 480I (interlaced) signal meaning there is 240 vertical and 240 horizontal and where they meet that interlaced, your cable connection with red, blue and green (component) will pass signal of 480P (progressive) 720P, up to 1080I depending on how high your monitor will output, but even if its 480P max that means you will have 480 pure lines of horizontal resolution and no lines will intersect causing distortion. Make sure your DVD player and cable box is set to the matching progressive setting for best picture. The Plasma should be 720P capable but thats a guess on my part.
I know you didnt ask but thought this info would benefit you......cheers

What then is the best component RGB video cable to buy?

As long as they are short and 75 ohm I do not think there is much of a differance in componant cable. They do not carry as much information as most other video cables.