Well, assuming you're not looking to drop $4k+ for the absolute latest hardware, and not having an actual budget to work with, and knowing your requirements are to "play games at max" settings and to have it last a few years.
Something running an Intel Core i7-6800K, an Nvidia GeForce GTX 1080, and 16GB of the fastest RAM the board will support (and preferably with open slots for future expansion) are what I'd throw into a machine that's upper tier at the moment without getting too far out of reach.
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Oh, and one more question: how much disk space does Windows 10 need? I've looked at some PCs at one shop and they were all built with two hard drives: a small one for the system and a big one for the rest of data. Some of these PCs had their small disks at 120 GBs. Is this enough for Windows 10 or not?
Those machines are likely outfitted with a Solid State hard Drive (SSD) and a traditional HDD. SSDs have read speeds of at least 2x that of a traditional drive. That means your machine will boot up very quickly, you won't be bottlenecked when gaming by your machine's inability to read data from the hard drive as fast as you need it, and the loading times of games on the SSD will be reduced. The downside is that at the moment they're still pretty pricey/GB of storage capacity.
Microsoft says Windows 10 requires 20GB of storage. Your average AAA visual blockbuster title requires around 50GB of storage. Therefore it is recommended that you actively manage where you put things when you're using an SSD as your main drive alongside a conventional drive, only storing programs (like games) that you know need that fast speed, while storing everything else (Office programs, music library, etc) on the conventional drive.
The "hybrid" drive that horus mentioned is a way to sort of split the difference. They use both types of technology and migrate your frequently accessed data to the faster technology while keeping the less frequent stuff on the older format. They're a good choice for someone who wants the potential to take advantage of a bit of a speed boost, but still retain the "fire and forget" usage style.
While the SSD will help keep load and boot times to a minimum, and I would recommend using one as part of a new high end system build, you do need to be prepared to do some manual work to keep things running smoothly, so the performance really won't be a ton of help if it means untenable quality of life changes.