Saturn,
No, I have not tested how DVD materials look like on my Yamaha when fed by a PC via RGB or DVI-D. Also, please note that I only tested the 75-feet cable with 480i souces; I do not currently have an HD source, or even a progressive DVD player! (or, is the X-Box progressive when connected with the component-out dongle???)
It is not a real "flat" cable. Because the manufacturer simply put together three 1/8" coax wires side-by-side parallely to make a tri-wire component cable, the entire cable has a rather "flat" shape (approximately 3/8" x 1/8" cross section).
As for your situation, I believe that as far as you use a coax-based RGB cable (rather than the usual twisted pairs), you will be fine with a 50-feet of analog RGB run. At any rate, I do not think there is any fundamental difference in picture quality between the analog RGB and the component connection. The signal bandwidths etc. are identical; the only difference is that the component connection carries Y/Pb/Pr signals, which is a simple linear transform of R/G/B signals.
By the way, I understand that DVI-D will not support a long cable run, correct?
