Doom 3 is not a good game, it is an average game. Doom 3 is Doom with great graphics and sound. Why would you want to buy a 15 year old game? The gameplay has not progressed one iota and, though the graphics and sound are top notch, the gameplay is vacuous and empty.
There have been too many advances in the FPS category to be able to call this a classic, or even a good game. I don't question the developers commitment to furthering computer graphics as they are simply the best indoor environments I have ever seen. Everything appears to have weight and texture and provides a solid base to start an immersive journey through a Mars base currently in crisis. The sound also can be merited with adding atmosphere to the game.
What cannot be put into question though is the lack of attention to detail on the gameplay front. Nothing has been done to give the appearance of intelligent opponents or varied gameplay. Die at the hands of a scary monster jumping out at you when entering a room and just reload from the last save point and go back to the room and take it out. The monster is in exactly the same place and does the same thing every time. Where's the fun in that?
Do yourself a favour, get the Doom anthology at a budget price and play the original with enhanced graphics and give Doom 3 a miss till it comes out at budget price. You won't feel as shortchanged about playing a 15 year old game that way.