All signal types include red, green, blue, horizontal sync, and vertical sync information. The more cables you have, the more that information is separated out and the more information can travel down the cables.
The PS2 does not have enough information travelling down the cables that you are likely to see any gain whatsoever in using a RGB/SCART connector vs. a normal Component connector. As well, the video will still be a VIDEO signal, not a computer/data signal. It isn't designed to be hooked up to a 1024x768 monitor and look killer, it is only designed to be hooked up to TVs.
The PS3 addresses this a bit more, but it may still be designed for televisions, not computer monitors. So, possibly, no XGA, SXGA, or UXGA resolutions, but 720p, 1080i, and 1080p will be there.