Batman Arkham Asylum I absolutely loved and thoroughly recommend. Not a FPS, but seriously brilliant.
Bioshock - will lose every spare second you can spare it for the time it takes you to complete it. Amazingly looking and captivating story.
UT3 - great for pick up, blast for half hour, put down times.
COD4:MW - Looks good, and I personally loved the storyline. My step-kids live for the on-line play.
COD:MW2 - Same as above, but a little better in every respect (single player at least).
Race Driver Grid - if you fancy a good driving game I haven't found better.
Burnout Paradise - if you fancy a fun driving game I haven't found better.
Left 4 Dead - seriously fun, although most reviews say L4D2 is better and should be the one to buy, but haven't got that so cannot confirm.
Crysis - if your PC is up to playing it, I personally like it. Know some people didn't like the storyline though, but I do. Currently playing through it single player again.
Anway, just a few ideas to get you started from my limited experience.