Well i don't know much about the xbox, but the green, red and blue cables your referring to are called component cables. They can output 1080p is some cases and with certain tvs.
If your talking about the xbox 360, then it'll improve visuals by a lot, from 480i --> 1080i, warranted your tv is capable of producing it.
HDMI is required to bring it to 1080p, but it won't be a huge difference unless your screen is capable of handling it, and your sitting pretty close to a large screen.
edit: component can bring it to 1080p, it'll just be analog as opposed to digital, and will require audio cables, including an optical for surround sound.