The Samsung Series 5 5000/5010 (2010–2011 models) has HDMI inputs. The Xbox 360 (all models) outputs HDMI (except launch 20GB units). This is the best connection.
Use Component Cable (YPbPr: Red/Green/Blue video + Red/White audio). TV must have Component input (rare on Series 5). Set Xbox to 720p/1080i. Quality is near-HDMI but analog.