You can get Master of Orion I and II from Good Old Games
for about six bucks, and it comes packaged with an emulator (DOSbox if I'm remembering correctly) so it runs right after install. If you haven't tried the second one, it's fantastic, and probably my favorite in the series. I still play it. GOG has a lot of old goodies that you might like actually- all cheap, all DRM-free (no disc or net connection required after you install, plus you can install on as many of your computers as you want).
The civilization series is also great, and you can find a free version called free civ
(starts download) that is based on the first two in the series. III is probably my favorite, only because of the interface, but I probably play IV more, at least until V comes out next month. Civ III and IV can be had for less than $10 together if you purchase the vanilla versions off amazon.
Moving outside the 4x genre, if you want a fun, quirky little game, World of Goo
might be up your alley; everyone I've shown this to has loved it, including my parents. Osmos
is also a good time. Both were DRM-free when I purchased, though that was a few months ago (I'm big on that aspect in a game). I've included links to the demos, the full version is identical in gameplay, just with more levels to enjoy.
Good luck searching, I hope you find something you like!