Well, based on those games, I'd say:
Crusader Kings 2
Published by the same guys who did Mount & Blade, this one's a
very deep grand strategy game that focuses on feudal politics and intrigue between warring noble houses. Basically, it's Game of Thrones in video game format. You assume the role of a single individual, the "head" of a medieval family, and go about forging alliances, amassing wealth, and claiming titles through marriage, diplomacy, and warfare. Sweet, bloody warfare! When you die (of old age, in battle, or poisoned by your greedy younger brother) you pass on your realm to your heir (IF you have an heir...) and then it's his turn to win glory for your family name, or lose it all and bring shame to his father and his father's fathers.
Medieval II: Total War
Like Rome, only better. If you can run Oblivion, you should be able to run M2
X3: Terran Conflict
Outer space flight sim and 4X sandbox strategy game. The next installment in the X series is coming out this year, so you'll probably hear more about it as time goes on, but for now, you could try X3:TC. If you can't get it to run, X2 is also an excellent game, and will run fine on any rig that can handle Morrowind.
Planescape Torment and Baldur's Gate
You mentioned getting the first two Fallouts; YES. GET THOSE. Then get Torment and BG, which are isometric RPGs in the same vein as the first couple Fallouts. They've been discussed at length elsewhere on the forums.
Hitman: Bloodmoney
Better than Assassin's Creed; you actually get to murder people and sneak around, instead of just being a glorified action game starring an urban free runner who goes toe to toe with hundreds of guards before fighting the Big Bad Boss.
Portal, HL2, and Sims 3 are all very good; of those, I'd go for Sims 3 as it's more substantial and will keep you occupied for longer, although bear in mind that it can be taxing on your machine due to the sheer size of the files and amount of content, especially if, like me, you've installed ALL the expansions. Also, HL2 and Portal both require Steam, which is a deal breaker. Archea mentioned
VtM:Bloodlines; I'll second that. It's an awesome FPS/RPG, more linear than Oblivion but still brilliant. Will mentioned
Civ II; I agree that the early Civs are better than the later ones, but I'd go for
Alpha Centauri over Civ II, or possibly a Civ-clone, like Masters of Magic (Civ II meets Magic the Gathering) or Space Empires IV (Spore's Space Phase, except done right).
It's probably just because I pre-ordered it a year or two before it came out and had been waiting for it since the first articles on it in PC Gamer when it was still on the Quake 1 engine, but I friggen love Half Life 1. Half Life: Source is their remake of 1 using the HL2 Source engine
