Anyone who has an old computer like mine (6 years old), knows it is getting harder and harder to find quality games that will fit your system.  Roller Coaster Tycoon's minimum requirements are only 16 MB RAM, a CD ROM Drive speed of 4x and 50 MB of free hard drive space, so it works great even on my obsolete system.  What is more, it is a fun game!  The first simulation game I became familiar with was Theme Hospital and, since I enjoyed that game so much, I decided to switch from the hospital beds and psychiatry couches to the roller coasters and concession stands.  Theme Hospital was a little easier to learn.  At first, I could not figure out how to get the rides up and running, but the tutorial soon set me straight.  Now I am having hours of fun with this game.