What are some must have Steam games?

Except Fallout 4? Well, the other Fallout games are must have too. xD And any game from Daedalic Entertainment! :D

Definitely the Binding of Isaac series, both the old one and new one, and their expansions! :)

In no particular order:

Secret of the Magic Crystals
Barbie™ Dreamhouse Party™
Hello Kitty and Sanrio Friends Racing
Kitty Powers' Matchmaker
Bad Rats: the Rats' Revenge

And every game from Humongous Entertainment and Digital Homicide.

If you like strategy games, I would recommend Total War. I can really recommend Medieval 2 Total War and Rome Total War, although they are pretty old (still not too old) they are really good. If you want some better graphics I recommend Total War Rome 2 (you could go for Total War Attila, but that one is a bit boring).

Another game I really enjoy is Brütal Legend, but it's not for everyone. People that are not into metal probably won't get the references it makes. Still a very good hack 'n slash game.

Left 2 Dead 2 (no need to play the first). Great game for who's starting the PC (if you have not played in consoles). Continues with many people online. http://store.steampowered.com/app/550/

1. Original Deus Ex (with mods like Revision)
2. Fallout: New Vegas
3. XCOM: Enemy Unknown
4. Trine and/or Trine 2; really gorgeous games w/ clever mechanics
5. Mark of the Ninja (and Gunpoint, perhaps)
6. Some game from TellTales Games, eg. The Walking Dead or The Wolf Among Us

It's almost identical to my PC =) I was building it for Witcher 3 mostly.

As for games - I don't know, there are too many choices which you may equally like or dislike.
If I were you, I'd opened all steam games list sorted by Relevance, narrowed it by tag and started looking from the top what could interest me.
I prefer open-world RPGs, Telltale-like adventures and logic games like Talos Principle.

1. Half-life
2. Half-life 2
3. Half-life 2: Episode 1
4. Half-life 2: Episode 2
5. Portal
6. Portal 2
7. CS:GO

Phantom pain i'm buying it when it goes on sale, gtav online.

First off, welcome to SG! (Prepare for a long post.)

For your build, as mentioned I'd build with the thought of expanding in the future. So this in my way is having extra slots on your motherboard, perhaps a second GPU slot, maybe even more room for more HDDs/SSDs. But your build looks fine and Fallout 4 ready, woohoo!

Checked your profile for what you own so I'm not redundant. My personal favorite is Fallout: New Vegas, which it says you own it but haven't touched. I HIGHLY suggest it, I'm really enjoying myself in it and the modding community is really great. If you ever want help with backstories or modding or getting past parts, I'll gladly help you that's just how much I love the game. Hitman Absolution, seen a lot of it and it looks fantastic with what can be done in choices. Shows you own it but not played, I suggest playing that too. I will wrap this up as shortly as I can now:
ArmA 2, despite the multiplayer limitations, you can find scenarios if you poke around, singleplayer is still fun.
Garry's Mod, has many modes and many addons, good little game to enjoy every now and then.
GTA IV, I've modded mine and broke it so many times haha. Great fun despite the horrid optimization, like with carmaggedon mod, LCPDFR, drivable tank mod, and character model changers. Probably best to not use all of these at once though, especially carmaggedon + anything if you want to play seriously XD. I'll shush now...

Left 4 Dead 2
Skyrim
Borderlands 2
Bioshock Infinite

Witcher 3 and CS:GO

The first is possibly the best game of the year and one of the best RPGs ever made, imo, the main is story is easily 100h, and to 100% it with sidequests and such you might take at least 200h.

100% Orange Juice >_>

One Finger Death Punch. Cheap, zero learning curve, fun as hell.

i'll recommend witcher series and mad max (i played it a bit on a friend's account and it's awesome <3 ).

I would change ONLY the PSU as your psu is a 80 bronze and for 20 usd more you can buy a 80 GOLD wich is 10% more efficient, seens little but when your using 500W, those 10% is a LOT.

Majesty: The Fantasy Kingdom Sim (One of the best RTSs I've ever played, and one of my all time favourite games).
Europa Universalis IV (Deep, complex and very rewarding)
Killing Floor (If you like fast paced co-op shooters, then this one is hard to beat. KF 2 is getting there, but still lacks content)
I have no mouth, and I must scream (I just played through this one, excellent storytelling but when i say that the game is mature, I really mean mature (and not "there's blood and nudity all over the place"))
Legend of Grimrock (Outstanding tile-based "blobber")
The Talos Principle (Beautiful, thought provoking and clever... from the team that gave us Serious Sam. This game managed to nab the top spot for games of this type from Riven on my personal "best of" list).

Arma 3, mate