If you're smart enough to have a hdd on your Xbox, then you can install the HD textures onto your hdd and get nicer looking textures than normal.
The reason this has to be installed onto the hdd is because the disc drive doesn't read at a fast enough bandwidth causing the textures to be at a lower quality. However, if it's installed onto the hdd, your Xbox can read the HD textures fast enough to be displayed normally.
This hasn't been confirmed for PS3.
Sadly, I can't play this on a computer which would be awesome.
(its a dell)