For strategy gaming, I would suggest:
Civilization II (I’m not a big fan of Civ III, though the patches do make the game better) If you can find it, it’ll be cheap.
Master of Orion II (Once again, I’m not a big fan of MOO 3, but that might be fixed with a couple good patches) There’s a jewel case only version out now for something like 10 bucks.
Rollercoaster Tycoon: Gold Edition (And I’m not a big fan of Rollercoaster Tycoon 2 either) Contains the original game and both expansion packs for something like 20 bucks.
Hey, this is interesting. All the games I like with sequels I don’t really like have had the sequels published by Infogrames.
Anyway, back to the recommendations.
Dungeon Keeper and Dungeon Keeper 2. Both are available in a 2-disc collection for, once again, something like 10 bucks.
There’s the Sim City line. Sim City, Sim City 2000, and Sim City 3000. There is also the massively popular spinoff The Sims (which I don’t like) and lots of older SimWhatever games, like Sim Ant, Sim Earth, Sim Life, Sim Tower, and probably others that I’ve missed.
That’s about all the strategy games I’ve played, but I know lots of people who like Starcraft and similar games. Not quite my cup of tea, as I prefer something a little more turnbased, although DK and DK2 are real-time.
RPGs:
I, at least, have this feeling like all the RPGs for the computer are generally MMORPGs (massively multiplayer online role-playing games). Personally, I hate MMORPGs, and you said you weren’t interested in online gameplay, so stay away from these.
There are RPGs out there for single player. I believe Square has ported Final Fantasy VII, Final Fantasy VIII, and Final Fantasy IX, which you might or might not like. There are some other games out there that are now in the bargain bin like Septerra Core, which I didn’t really care for.
Honestly, most RPGs I play are console-based. I don’t really play them on the computer.
There’s always also the declining genre of adventure games. What I would look for would be:
All four Monkey Island games. My copies of The Secret of Monkey Island and Monkey Island 2: LeChuck’s Revenge came bundled with a $15 copy of The Curse of Monkey Island. Escape From Monkey Island, being the latest game, is probably going to be the most expensive, but I would still expect it to be pretty cheap. (I just realized what a weird name Monkey Island is, after typing it all those times, and I guess I really should put in the trademark symbol.)
Other fun games in this category are Grim Fandango and Sam and Max, both of which were also released by LucasArts. Starship Titanic is another fun game, as it’s based on a throwaway line in Douglas Adams’ Hitchhiker books and had a nearly simultaneous release with a book written by Terry Jones (who also provides the voice of the parrot.) A darker game would be Blade Runner, which is quite fun and captures the feeling of the movie very well.
Heck, for that matter, see if you can track down LucasArts other old adventure games, like Loom, Zak McCracken and the Alien Mindbenders, and Indiana Jones and the Last Crusade (the rest of the Indiana Jones games just aren’t that good.)
There are so many games out there, and a lot of them are older. Unfortunately, the older they are, the harder they might be to acquire legally, and I’m not going to bring up how to get them otherwise here at the SDMB.