I answered this question in an earlier post, but simply put: at this end of the market (budget) you just need to consider each board individually for the features you want/need and most importantly: the price.
You're right, A520 boards are newer than B450, but they're technically a lower tier product:
- B450 was the midrange option for Zen+ and later, Zen 2.
- A520 was the budget option for Zen 2 and later, Zen 3.
If you want the PC to last for a long time, then I'd recommend you get an entry-level B550 board, reasons being:
- They have dropped in price a lot and you can get a pretty good one for £100-£120.
- PCI-E 4.0 could be useful later, particularly if you buy a lower-end GPU (like 6500 XT) which tend to lose more performance in older gen boards.
- The VRMs tend to be stronger, which could save you a headache if/when you upgrade the CPU.
- You usually get 2x M.2 slots and that's the way storage is going.
You're also more likely to get the CPU working out of the box with A520 or B550 (i.e. no BIOS update required), but I'd expect most B450 boards to have been updated by now.
On the other hand, if it ends up: you can have a good board and a bad CPU, or a good CPU and a bad board, then I'd take the latter option.
My honest recommendation is get a Ryzen 5 5600 (not G) and a B550 board, because I think that will give your PC the longest usable life, at the lowest relative cost. Since the small savings on the CPU and motherboard now (e.g. 4500 and A520/B450) might come back to bite you later.
It is fine, just get a good deal, but with the low price of memory right now, I'd recommend you get 32GB.
I don't know if AMD/NVIDIA are preferable for emulation, but generally they're comparable (5600G and GT 1030), with some games doing better with either option. I'd probably keep the GT 1030 and get a 5600 non-X, but it is your call. Removing the GPU does have some advantages, like the option of a more compact build and lower idle power consumption.
