So, I'm normally a console gamer, but I did have a good PC 10 years ago, and I just build a pretty good PC now. What games should I buy? I have most multiplatform games for consoles, so don't name stuff like FFXIV or Battlefield because I'd rather own them for PS3 (even though my PC is more powerful).

What I am interested in is stuff like SimCity and Starcraft 2. Games which will never appear on consoles and need a mouse to play. I'm going to buy Starcraft 2 today, and I'm holding off on SimCity because it doesn't have a "Classic Mode."

However it's been 10 years, there must be a lot I've missed. Here is what I have

Counter Strike Source - Love it
Fallout 1 & 2 - Love them
Half-Life 2 - Only bought it for CSS, but I do love it
Sim-City - Love it
Warcraft 3 - Love it

What I'm looking to get so far

Civilization V - Waiting for a Steam Sale
Company of Heroes - Considering
Diablo 3 - When it becomes cheaper
SimCity - Thinking about it
Starcraft 2 - Buying in a few minutes

I'm really looking for SimCity like games, or games which are just not possible on consoles. I still might get SimCity despite the flaws.

P.S. I suck at RTS games, so I'm limiting myself to Starcraft II

Hmm, let's see

Titan Quest or TorchLight if you like Diablo-esque games.

The Witcher 1&2, for WRPGs.

There is CitiesXL which seems to be a good city builder like SimCity.

Just to name a few.

League of legensd/Dota 2 are free games. They are both amazing and they are a definitive part of the pc experience. If you liked warcraft 3 they are both a must. 

Total war games are amazing i would suggest starting with Rome 1, you can get it for 5 dollars on steam. Or for more while getting an expansion and barbarian invasion. Then you can continue down the total war series with games like empire/napoleon and shogun 2.

And get The Witcher it is cheap, has value and is a very good rpg though it lacks in the combat department. Then you can get witcher 2 later on which was a significant improvement. 

Tropico 3/4 and Cities XL are quite like Simcity, Tropico being simpler, Cities more complex. Both have distinct styles, Tropico has far more personality though. Tropico 4 also has a pretty good map and scenario editor if you're interested in that sort of thing.

Definitely buy the 2 expansions for Civ, it feels incomplete without it.

Anno 2070 http://store.steampowered.com/app/48240

The Total War series is pretty epic, and Europa Universalis is kicking ass as well. I think both can be defined as Grand Strategy games, much like Civilization.

Arcanum: Of Steamworks and Magick Obscura ($6 on GOG), if you missed it, made by same people who made Fallout 1.

Go for Torchlight 1/2 if you like Diablo, it's much better Diablo than Diablo 3.

The Witcher 2 I have for 360, so I only need 1 for PC.

Got DOTA 2 and League of Legends.

I actually never played Diablo, I just hear it's good and it's from the makers of Warcraft and Starcraft. Should I play 3 first, or 1 & 2 first?

I'll look into Tropico and Cities XL. This could be exactly what I want. I was turned off Tropico because the 360 port is bad apparently, but I'll look into it.

If I like Civ, I'll look into Total War. I don't want to buy more games then I'll play, I want to beat Civ first.

What exactly is Anno 2070?

What's GOG?

http://www.gog.com/game/arcanum_of_steamworks_and_magick_obscura

Good Old Games, owned by CD Projekt RED, makers of Witcher.

Just get a few games for now and wait for the Christmas Steam sale.  Then you will be able to get everything you want for the price of one game ;)