Smoove17: Earth 2140, Total Annihilation, Total Annihilation: Kingdoms, Dark Reign, StarCraft, WarCraft I and II, the early Command & Conquer Titles, Dune and Dune 2000., Age of Empires I and II. (The bold written titles are available right here on gog)
I'd drop Warcraft 1 and Dune from that list, due to their crude controls. Plus, Dune, even with the latest patches, had the "maximum number of objects" bug which made some of the final levels almost impossible to play, unless you made sure you kept killing excessive enemy units all the time so that the limit would not be reached.
It was kinda funny when the limit was reached, you'd have lots of friendly and enemy units on the screen, including turrets, but they were unable to shoot a single bullet. A massive staring contest.
If you want Dune, Dune 2000 is much more pleasant to play. Dune 2000 resolution (640x400) has some display issues with widescreen monitors (e.g. it isn't displayed full-screen, or some odd artifacts), but since the OP has a very old PC, probably it has 4:3 display anyway.
For Wacraft 2, BattleNet edition is the best, improved, version. Runs fine in 64bit Win7, albeit it needs the "funky 256 colours" fix.
I just wonder how slow or bad the OP's PC really is, if he feels it will not be handle almost any 3D RTS games. I've played e.g. Age of Mythology, and I think also Emperor: Battle for Dune 3D RTS games on something like an ancient IBM ThinkPad T40 laptop (which entered the PC market something like 10 years ago?), and at least AoM ran just fine on it, as far as I can remember. Probably Emperor too. I'm pretty sure something like Warcraft 3 would run passably on it as well.