'Retro' game for olde farte

[LordGrover]

Back in ye olden dayes, when I had my master system and then megadrive I used to game quite a bit. Now, in my dotage I quite fancy having a go again, but I can't find any games that I really fancy. Preferably on PC, maybe OS X - have both but no console. Not too demanding on graphics as don;t want to fork out on GFX cards and the like.
For instance, I used to like games like Populous and Dungeon Keeper as well as FPS like Quake and Quake II - anything similar available today? Nothing too frantic though, eh. Nearly forgot RPGs like Final Fantasy were good too.

[LordGrover]

Thanks for suggestions. Like the look of Portal 2 and Dawn of War.
Have downloaded Steam, which in itself looks like a useful source.