First of all, I'm going to assume that you are fairly young and most of your FPS gaming experience is with modern games like Halo. Marathon was extremely advanced for its time, but the original first game is nearly 10 years old now. The weapons and monsters are drawn using sprites (cartoon-like animations) since true 3D rendering engines didn't exist back then, and the textures are very crude by today's standards. The good news is that you don't need a very powerful computer to run any of the Marathon games. They will run fine on virtually any Macintosh computer built within the last 8-10 years. If you have a Mac or have access to one, you can purchase all three games with a few others thrown in from the Bungie store in the form of the Mac Action Sac for only $10. This is by far your best bet.
Now I'm going to go out on a limb by assuming that you probalby use a PC. Converting these games for use on a PC is not a trivial effort, and we can't do much other than to help you because of copyright limitations. However, there is a conversion of the original Marathon game for Aleph One SDL (M1A1 SDL), which will run on just about any PC. Best of all, it's completely free. Head over to the Marathon Open Source Page to download the latest Windows build. Unfortunately, the link on this page to M1A1 seems to be broken, but you can pick up a copy from the Lh'owon Ar'kives at Fireball and, while you're at it, be sure to pick up the M1A1 for SDL Script Update. This should be enough to get you started and for you to decide if you like Marathon enough to go to the trouble of converting the other scenarios to work on your PC.
Again, the thing that makes us all fanatics is that Marathon has the best integration of story and gameplay of any computer game ever created. It's a great sci-fi thriller. And besides, Mac users tend to be fanatics anyway.
