If you were into the LucasArts games, you should definitly pick up Grim Fandago and any of the Monkey Island series you missed. Another great adventure game that is newer is Fahrenheit aka Indigo Prophecy. Might check up on the specs, but I think you could get it running. And of course, the new Sam and Max series which you can buy online and download right away (Google it if you're interested).
As far as sim goes, theres tons and tons of stuff to choose from. 'Sim' is kind of a wide genre. Have you tried Civilization 3? Its a turn-based "civilization" sim letting you run a country from the stone-age up till today. If you're looking more for a business type sim, the Roller-Coaster Tycoon games are pretty solid and fun. If you want something more focused on war and battles, the Total War (Rome Total War, Medieval Total War, etc) are really fun but you need to have a certain ammount of hard-coreness about you.
There are really tons of good PC games out there, and 98% are backwards compatable! If none of these really appeal to you, let me know what you'd be more interested in and I'll get you some titles to look into.
Happy gaming and Merry Xmas
GF