Red Faction is awesome on PC but don't bother with Red Faction 2 because I thought that sucked. RF1 is available as a budget title now and is still an awesome shooter. Half Life is another excellent choice, if you haven't played HL1 then I suggest you drop everything and go find yourself a copy of it. If you get the game of the year edition it comes with Blue Shift, Counter Strike and Opposing Force, enough gaming content for a long long time. Well worth the money.