Beneath the Steel Sky is one of my favorite adventures ever. It beats most sierra adventures, IMO.
Jedi Knight: Dark Forces II isn't DOS and it has a great community, still. Go over to Zone.com. People still make many levels and mods for it. If you think the game is boring, download some of the mods from
http://www.massassi.net . It's definitely my favorite PC game ever, if you use the mods (I'd suggest Space Soldiers 3 [equipped with Desert Eagles, Pyro Ion cannons, pipe bombs, blood, and much more], Killer Weapons Pack [sniper rifle, and some other cool guns], Explosive Weapons Pack [self-explanitory], Saber Battle X [normal sabers, long sabers, double edged sabers, new force jump, and much more. It's more than just a "saber mod"]
And, I've been looking for Dark Forces forever. I finally found it but it didn't work on my computer, go figure. I would keep it, if only so that you can say you own the whole series.
Oh, and that is of course if you own Jedi Knight II: Jedi Outcast. If you don't, you should. It's pretty new (came out in 2001 or 2, I don't remember). It's still $50 in Wal-Mart though.
I've never played Terra Nova, but I'd sell the others. They aren't good.
