On behalf of xtisme and myself, I say to wait for Medieval 2 Total War. That game should make my drawers ultra wet.
Morrowind and Baldur’s Gate : Throne of Bhaal are good single player RPGs that will work on older computers. Here is a link to The Ur-Quan Masters. I like the orignal Homeworld; never had the chance to play the sequals.
Destroy All Humans is fun, as is God of War.
Destroy All Humans is hillarious.
Star Wars: Republic Commando was lots of fun on the XBox, dunno if it’s any good on the PC (but as long as they kept the smooth squad controls and the orgasmically fantastic music, you should be fine with it). Of course, I’m the guy who laughed out loud when he heard the main character’s name, and was the ONLY one in the room full of people who got the joke.
I’ve been trying to get back into Homeworld, but it keeps making me think of how much cooler it would be if it was BSG (since Homeworld basically IS BattleStar Galactica). Sadly, I have to get Homeworld 2 if I want to get in on the BSG mods.
Age of Empires 3 is another one I’ll throw out there, depending on the specs of the computer that would be running it.
If you’re looking for something single-player that’s truly old-style and won’t tax your system, there’s always Angband.
Yet another odd SDMB coincidence. I have a sum total of no games (excluding those in Windows) but on the weekend had a flashback to my kids playing the original SimCity and bought SimCity4. Hope I have as much fun as I remember the kids having, “Hey Dad look at this…”
You’ll love it, but make sure you get some of the mods from Simtropolis. There’s one mod in particular for pathfinding that makes the game much more playable at higher densities.
Without that mod citizens prefer driving on the shortest route rather than going a little out of their way to take a faster route or mode of transit (highway, subway, bus on a highway, or train). It makes your citizens European rather than American, basically. 
If you like this sort of game, I highly suggest Abe’s Odyssey. A great 2D Side-scroller that was a blast to play.
Be careful. You can easily fill up an 80-gig HD with add-on stuff from Simtropolis. 
(Do a search there for ‘Simgoober’ files. You’ll be amazed at his handiwork.)
I love SC4. I build the cities, then my 6yo son drives the school bus through them in a manner that would mortify Mr. Bus Guy.
My ‘go-to game’ is Transport Tycoon Deluxe. I’ve had it forever, and I still drag it out and play it when I burn out on everything else. Not real fancy, but once you get a good rail system going, it’s mesmerizing to watch. And hey, now you can get it FREE!. (Get the TTD patch, too. Link’s on the Wiki page.)
Yes, I agree, SimCity for a solo game.
problem i have with SimCity is that it’s very repitious and time consuming… myself, I prefer quick games… I used to play Diablo alot, but there was a point that it just became tedious.
I’m still playing RedAlert2:Yuri’s Revenge… I usually go against 2 AI opponents in a skirmish, usually takes 30 minutes to defeat them, and I gets to blow stuff up.
I have yet to find a new game that gets that kind of mileage… I don’t want to spend days/weeks/years developing something to go smash stuff… I want to smash stuff from the word go, and keep finding new ways to smash stuff…
Have you seen Locomotion?
DiabloII?
How is this going to be different than Sim-Earth with better graphics? It sounds interesting, but I was soooooo disappointed with Sim-Earth.