Ryzen wants the fastest RAM it can handle, especially if you want better internal GPU results. I just got some Geil 2x8GB 3000 for about 120 USD from Newegg. The big thing with the B450 chipset is you want to check to see if your RAM is single ranked or dual ranked. Dual ranked RAM will reduce your maximum RAM speed you can actively use on the motherboard as you add more chips.
The 2600X is basically the best binning of the 2600 series. The 2600 are cheaper but will probably overclock well. The 2600X will probably not overclock well since it is already best in its class. So it breaks down to, do you want to overclock yourself or have AMD do it for you? That's the difference.
On a side note, the 2600X is considered one of the best gaming AMD CPU right now because of its higher base/boost clock. Also, it doesn't have so many cores it gets in the way of a game running properly. It also still has only one NUMA core.