I recommend the Orange Box if you haven't played that before as its by far a must own game for any PC gamer. You'll get three great games including Half Life 2, Half Life 2 Episode 1 & Half Life 2 Episode 2. It's a very arcade style shooter like Doom and it's a lot of fun. On top of that you'll also get Team Fortress 2 an online shooter and you'll get Portal a first person action based puzzle game with great humor. Both range from $30 retail to $20 digital on Steam and run great on older hardware.
Two other games you might be interested in are Quake 4 and Prey both based on the same game engine as Doom 3 (id tech 4). The Prey story line however is much more engaging.