Mass Effect is a nice buy at the moment if you like FPS and RPG's. It's a good mix of the two, and while it may not be the most mega awesome game in existance, it got alot of good praise for good reasons, fun to play, and cheap to buy.
The Gears of War series, naturally, would be the next step. I've never played them myself, but I'd probably be lynched if I did not include them.
Assassins Creed is quite a neat game aswell! Beautiful visuals.
Now, the one to definatly avoid is Halo 3. I loved the first two, and found the third very, very... very overhyped. It may be fun online, though, but, I found everything else to be a total bore, poor graphics and singleplayer, and, to be honest, weak mechanics.
1500pts Celestial Lions
1000pts Warboss Judge Skull'Umper
500pts Vostroyan & Cadian 119th