I haven't really used my Xbox much recently, especially since the HDMI outlet on the thing broke - I've tried about 5 HDMI cables, including new ones I bought after it happened, before anyone makes any suggestions - but I'd recommend the Mass Effect series, Halo series, Battlefield 3 and the Portal games as definite things to grab. I'd also recommend Alpha Protocol if you like RPGs and Deus Ex style games but do be warned that it can be glitchy and got a lot of negative press, mostly due to people playing it as an action game rather than a pretty deliberate RPG. Main hint is to actually keep your crosshair on someone for a few seconds for every gun type except the SMG, as they all get bonuses for it.
_________________
Power corrupts. Knowledge is power. Study hard. Be evil.