Dark Reign raised the bar in the RTS genre.One of the earlier Windows 95 and Direct X games, it ran well even on my old 486 DX66 and the advanced waypoints combined with the ability to tweak the AI of each unit - or groups of units - made for endless variations in tactics.
It only runs at 640 x 400 resolution as far as I can tell - which may or may not be a concern for you. If you have an older PC, or no 3D accelerator card - or a laptop - then it's definetely worth a look.
Contact MiGMan with your experiences regarding this sim.
Upgrading the buildings allows you to build more advanced units.