FPS: FarCry, Call of Duty, Half Life 2
RPG: Morrowind, Knights of the Old Republic (and if you've played the first, you'd want to play the Sith Lords too, even though it is not as good).
Strategy: Rome: Total War, Medieval: Total War - Viking Invasion, Europa Universalis II, Crusader Kings
R:TW has a great modding community (in fact, the game wouldn't be half as good without it), and there are some pretty major TC's coming out in the future, as well as all the excellent mods - both major and minor - that are already out there. Ones to look out for are: Napoleonic: Total War 2, Chivalry: Total War, Fall of Rome: Total War, Europa Barbarorum, Rome: Total Realism (+ submods), SPQR, Middle-earth: Total War, Citadel: Total War, and The Fourth Age: Total War (check sig). There are many others out there, too many to name.
Lagus: I buy a lot of my games from play.com. They tend to have a lot of bargains, and they have free shipping. Also a good place to buy books and dvds. They currently have M:TW - VI for 17.99 (Battle Collection) and EU II for 7.99.
- Adanedhil