Hi all, I realise this question probably gets asked once every five minutes or so, but I'm a complete noob to PC gaming, and am really unsure where to start. For my first build, I won't be playing anything particularly crushing, so I've around £500 to spend (with a little leway), and might upgrade in the future if I need to. Looking to build, so if anyone could point me in the right direction, that'd be amazing.
The component most often upgraded in PC gaming is the graphics card, so you need to think about a PSU that can handle them. Unfortunately preparing for upgrading can add a bit more cost in some areas. As another example you could pair a cheap motherboard and cheap CPU to begin with, but then better CPUs will struggle in that cheap motherboard.
Anyway, to make things simpler - 450W PSU will run "good" cards, 550W "very good" cards, 650W "enthusiast" cards. What does that translate to with current AMD graphics options? It means 450W will run RX 570s and some RX 580s, but will struggle with some RX 580 and 590 models that can spike up to 300W and for which you'd want 550W PSU to account for everything else that uses power plus a little headroom. With Vega 56 and Vega 64 you'd need 550w or 650W depending on whether you undervolt them or not. My advice - get a 550W at least. That will cover plenty of excellent graphics options even if not the very top card (that tends to cost a bomb anyway).
A motherboard like the B450 Aorus Elite in Dan's spec above would be a decent choice. Not too expensive but not dirt cheap and useless for Ryzen 8-core CPUs. And absolutely fine for that 2600 6-core which you probably won't need to upgrade for years until it's time for a whole new system.
And here I'll add something counter-intuitive. You may have heard that when putting together a build you should spend a lot on the graphics card if it's for gaming. There can be exceptions. Since you are a self-confessed PC noob, I would assure you that you would have fun with an inexpensive graphics card to begin with. And if second-hand then obviously you don't have to spend as much to obtain something very capable. I've been running with a 1GB GTX 750 and still having fun on Fortnite and GTA V, even after getting used to a faster GTX 970 4GB that died, while waiting to see what new graphics cards would bring. And there are much better options than a GTX 750 anyway even with your limited budget.
So I'd go second-hand for your first graphics card. On the 'bay, some GTX 970 and RX 580 are selling for £75-100, for example. Even a slower 1050Ti 4GB for £50 (if you can spot a good deal) would be great to start with. OcUK also have B-Grade cards in the %DEALS > Clearance section and you can check those out too but note that in most cases they don't list them with price reduction compared to new, and you have to contact them and ask at what price they are willing to sell a particular B-Grade item.
Solid State Drive (as included in Dan's spec) should be a must for your OS and programs, even if it can't hold many games. They are very affordable now. Add another drive, whether SSD or mechanical, later on.
RAM just go with 16GB RAM, you're going to want it anyway in the long run. But if budget doesn't allow, reduce to 8GB RAM and add more later.
"Paul's Hardware Windows 10" (youtube) has advice for your Windows options.
My basket at Overclockers UK: Total: £554.46 (includes shipping: £12.60)
Spend £75 on an above mentioned second-hand/B-Grade graphics card, and you have a very nice system for £625 with a well built Phanteks budget case. Or around £575 if you shop around. Hopefully that will be the "little leeway" you mentioned for the £500 budget. Otherwise, it can be £525 with 8GB instead of 16GB RAM.