Forgot to mention the hard drive. Duhhhhhhh
You must get a solid state drive (flash) for your system and games. Starting up your computer takes literally 5 seconds. Games and programs start immediately.
For brand, this is where Korea does shine. Samsung purchased western digital which was the premier maker of SSD, so now Samsung is the premier brand.
I have two 256 gb Samsung 850 pro SSD and a 1 TB HDD. But this is not necessary. Many people will save money by getting a 256 or even 128 gb SSD and then getting a 1 TB HDD (traditional hard drive that spins) for storage (photos, music, movies, etc stuff that you don't need quick performance for). And a 500 gb or 1 tb HDD only costs like 100 bucks depending on brand. And you can get them on sale all the time.
Again, that's the beauty of pc building, you can customize to your needs. A 128 gb SSD will fill up super super fast so that's why an extra HDD is good.
Samsung has 840 vs 850 and EVO vs PRO. So there's four possibilities: 840 evo, 840 pro, 850 evo, 850 pro.
In order of quality: 840 evo, 850 evo, 840 pro (just barely, since there has been known issues with the 840 pro), then the 850 pro.
You could get the 840 evo and probably not even tell the difference, but the pro series will definitely be better. For example, the pro series has a 10 year warranty I believe whereas evo series only has 5. Also pro has longer longevity with its rewrites. For 850 vs 840, the 850 has V-NAND (flash chips are stacked vertically to improve performance). Again, this is very miniscule for the casual user.
The 850 evo was on sale on amazon for $100 (almost $80 off) and still might be so. I recommend you get an 850 evo regardless 256 gb then 1 TB HDD (500 GB if you don't have much files). Then, put all your windows and games on the SSD. Once it starts to fill up, buy another SSD.
Honorable Mention: Crucial MX100 250 gb. IMO it's better than the 840 evo because it performs just as good (worse warranty however) and costs only like $120 even when not on sale. The normal priced 840/840 evo are usually around $160-180.