What cables are you using? HDMI, the Component cables that came with the xbox 360?
Yesterday I brought a new xbox 360 sand a 22inch tv the Samsung p2270hd when playing games the image isn't smooth at all like there's no anti-aliasing think you call it staircases? Its set to 1080p ive tried setting it to 720p 1080i etc etc no difference. Ive had gamemode on and off still no difference. Only difference is if i plug in the VGA cable it looks better but still not acceptable i guess.
The weird thing about this problem is on branded tvs like LG and SAMSUNG that ive had in the past it looks bad but on non BRANDED rubbish hdtvs my xbox always looked better and smoother edges around everything. I used to have the xbox pluged into a old DELL monitor at 1024 x 768 it looks ALOT BETTER! Can someone explain this as its always happened to me with different xboxs cables etc so it cant just be me!