Doesnt really matter, most of the xbox 360s on the market dont actually support up to 1080p on games, max it can do is 1080i on dvds.
people will dispute this, but its true. just look on the back of all of your games, they'll all say primarily 720p. I got the same quality through the HD-avi cables as the xbox VGA adapter i used to connect to my old CRT (was a pretty damn good CRT though)
If you want to see real quality, go ps3 but yeah
edit: but i have noticed slightly better colour vividity on the same monitor through different cable means, but i dont think the quality was noticeably different
AND people will moan, exert from wiki:
Xbox 360 Core
The Xbox 360 Core was an entry level Xbox 360 which was later replaced with the "Arcade". Although available at launch in other regions, it was not available in Japan until November 2, 2006. The Core system came bundled with composite video cables, capable of only SDTV resolutions. It was however capable of the same HDTV resolutions (up to 1080i) as the other models when connected to a separately sold component cable. In October 2006, 1080p support was added for all models in a system update, including the "Core" using either the component cable, or the new VGA cable (although 1080p via component was not widely supported by televisions). It may also utilize a separately sold Xbox 360 hard drive. Unlike all other SKUs, it shipped with a wired version of the Xbox 360 controller, instead of the wireless version found in other SKUs.
check the back of your games still, i bet im right