I was very happy when X-Wing and TIE Fighter made their way to GOG last year. And I'm happy again today, because they, along with X-Wing Alliance and X-Wing vs. TIE Fighter, are now on Steam as well.
The X-Wing Special Edition on Steam includes both the original 1993 DOS version and the 1998 Windows release, as well as the Imperial Pursuit and B-Wing expansions. Likewise, the TIE Fighter Special Edition features the main game and the Defender of the Empire expansion in both 1994 DOS and 1998 Windows flavors, and the Enemies of the Empire expansion that was added to the CD-ROM version of the game that came out in 1995.
X-Wing vs. TIE Fighter includes the Balance of Power expansion that adds Imperial and Rebel campaigns to the game that was originally designed primarily as a multiplayer experience, and X-Wing Alliance comes as-is, because it's the only one in the bunch that didn't actually get an expansion or post-release upgrade.
The system requirements are pretty light, as you might imagine, but bear in mind that the 1998 releases of X-Wing and TIE Fighter require a joystick or controller. The original releases can be played with a mouse and keyboard, but why would you want to do that? Luke didn't blow up the Death Star with 101 keys on his lap, and neither should you! X-Wing, TIE Fighter, Versus, and Alliance are available individually for $10/£7, or all together in the $30/£23 Star Wars: X-Wing Bundle.