Oh boy, where do I start.
Hi, professional here. Please don't ever, ever, ever, ever, ever use UserBenchmark for comparing anything. They have a very clear Intel bias, and have been banned from both r/Intel and r/AMD on Reddit as well as other PC communities for being complete bullshit. When AMD started curb stomping Intel a couple years ago, they changed their rules so that Intel still wins, no matter what the actual performance is.
Memory amount is, in absolutely no way, a measure of performance. However, for gaming, 16GB should be your target minimum in 2020. That said, my spare R5 1600AF machine that I have hooked up to a TV has 8GB and still plays everything modern just fine. Also, the 8GB machine you posted has faster memory, and Ryzen loves fast memory.
The 200 mhz difference between the Ryzen 5 2600 and the Ryzen 5 3600 isn't the reason the 3600 is better. No one is going to notice 200 mhz difference on a CPU. The real reason the 3600 is better is because the Zen 2 architecture it is running is much faster than the 2600's Zen+ architecture. This means that for a given clock speed (lets say both were running 3.5 ghz for simplicity), the R5 3600 can do more work for the given speed its running. This is called "Instructions Per Clock", or IPC. However, for casual gaming, either one is just fine. My personal gaming rig has a 2600 in it.
GPU memory at the prices you are looking at isn't really important. We are talking lower end 1080p gaming either way, and the 1650 and the RX 580 perform pretty closely, with the 580 being just barely ahead. GPU memory only matters at higher resolutions where assets are larger in size. What matters is raw compute.