This.
Choosing your own components means you get exactly what you need & not someone else's approximation of what you need. And beware of buying any pre-built system if the supplier doesn't give a brand & model name for *every* component, because I've seen supposedly high-performance machines that had only the stock cooler, or were fitted with memory sticks from the bargain bin. There's not much margin in building computers, so there's a motive to cut corners & reduce costs. Alternatively, you can expect to pay a premium for a machine that's properly specced & built, if only because the builder has to earn a living.
When I built my current computer in 2010 I deliberately chose the i5 2500K over the i7 because with a decent overclock it would give me 95% of the i7's performance for about 60% of it's price.
The i7's hyperthreading may have been useful when rendering video, but the i5 (when used in conjunction with my GFX card & SSD) still manages to process & save video at around 10 minutes per gigabyte, which is quite bearable.
Everything I've read indicates that if gaming is your main focus, an i7 would be a waste of £100; I was also preparing to get an i7 before I was told that an i5 is all you need for gaming. If you're multitasking and doing video editing and so on then go for an i7.
I Agree, I was aiming for an I7, until I did lots of reading, My I5 Whoops "donkey", particularly when I've used a GELID Tranquillo CPU cooler, Means my I5 3.3Ghz, is clocked to 4.2GHz, and Playing Counterstrike GO, the 4 cores only reach about 60% utilisation with the CPU clocking upto 4.0ghz tops.
Also whatever you do, you're better off with £100 spent on say a 450W quality PSU than £35 spent on a cheap 650WPSU. I've also seen this first hand, and had to explain to my mate why his new top notch GFX card was giving poorer FPS than his old one. It comes down to sustainable power per channel of the PSU. A quality 450W may supply 450W peak down 3 channels individually and maybe 350W continuous down 3 or 4 channels, a cheap 650w will only PEAK at 650w on all channels combined and it's sustained output might be 500W over all channels ie 167W per channel on 3 channels or it might have 2 channels only. (More complicated, but that's the easiest way to explain it.)
Also a quality (asus/Gigabyte etc) nvidia GTX760 with 2GB Graphics RAM, will probably far outperform a..... errm thinking... SPARKLE GTX780 with 3GB of RAM.
Quality is just that, quality. Also the Asus and Gigabyte, will probably come with overclocking software and you'll achieve much better results.
On your case ensure airflow. A quiet high-CFM airflow at the bottom front of your case, and either a single same or higher spec at the rear + PSU or 2 x slightly less/same spec + PSU. And if possible venting/grille over your CPU/GFX from the side of the case. I've found vents over these two key positions and a negative pressure, (ie more extract fan power than intake fan power), allows cooling to be dragged in through the side vent direct to the CPU and GFX.
I could be wrong, I've only been building gaming PC's for 15 years, and done the watercooling etc, finding that air cooling is more than sufficient to compete with its extremely expensive counterpart. Although done right water cooling looks the dogs danglies.
Finally, research parts, see what others use, see what problems people have had, you may find a certain quality gfx card, doesn't work so well with a similar quality motherboard, but a different mobo from the same manufacturer, and even the same family of motherboard with different BIOS version, or other features may work well.
