Thanks for the replies, sorry I forgot to say I'm mostly curious what sort of price it would cost. Let's say a budget of around £2000, but I'm also curious how expensive these can get and what would improve as the price goes up? Thanks. :]
You could probably spend £30,000 on something that was technically a PC (but would usually be called a workstation) if you really wanted to. Maybe more - I'm guessing a bit at that level. You can spend £5K on a single graphics card if you really want to. It would be ridiculous and completely unsuited for use as a gaming PC, but you could use it as one and you did ask for "how expensive these can get".
For gaming, improvements would be marginally useful above a certain cost. I'm out of date on hardware, but I'd be surprised if that cost is over £2K even with the eye-watering cost of PC hardware (especially graphics cards) today. At the high end, you can easily pay double for a performance improvement you won't notice in real use.
Also I don't really understand Hertz. Similar to FPS..? I'm not sure what the standard is and what is considered 'good'.
Similar, but different

Roughly, FPS is how many frames per second the computer can generate and Hz is how many image changes per second the monitor is capable of.
Do we still use 60hz monitors? I've not payed attention to this for about the last 15 years. :V
Some people do, but nowadays good quality monitors are usually also capable of higher refresh rates. I'm using 75Hz because when I bought a new monitor there was a specific model that had a remarkable price/performance ratio and that was limited to 75Hz. For me, paying half the price for a large, responsive monitor with the resolution I wanted and good picture quality was better. Other people swear by 144Hz and wouldn't willingly go lower.
I see that water cooling has been suggested for noise levels and that does make sense, but you can get a modern aircooled PC down to imperceptable noise levels if you choose the parts appropriately. Mine is inaudible to me at a distance of about 1m (it's to the side of my desk, on the floor). After I built it, by far the loudest noise was the hard drive being kept in a ready state. Not in use - that was a bit louder. Just in a ready state. So I fitted SSDs instead

Since you also specified "fairly fast to boot/load", you're going to want SSDs anyway. The difference in boot time is extreme. It's not
so noticeable in loading games, but it's
really noticeable in booting. From cold, by the time my monitor has come on I'm already at the desktop login screen and I'm using SATA SSDs, which are much slower than newer standards (but also much cheaper).
Admittedly, I had to shop around for a case wide enough to fit my CPU cooler in (it's ridiculously big) and it was rather expensive for an air cooler but the size and cost of the cooler and the fan on it mean that the fan can cool efficiently while spinning very slowly. Cheap, effective, quiet - pick 2 of 3

Right now, in a room that's currently 22C, my CPU (which is permanently at maximum boost speed) is at 29C with a fan speed of 490rpm. My system fan is at 770rpm, PSU fan is at 0 and graphics cards fans are at 0. The highest reported temp is 46C on the hottest part of the motherboard, which is well below anything to care about. If I lie on the floor and put my head next to my PC, I can hear the case fan whirring slightly. Of course the fans spin up during gaming, but even when I stress tested everything it was still quiet even to me and I'm fussy about noise. So I can confidently say that you can get a very quiet PC with air cooling. It's some fuss and some expense, but it's possible.