I wouldn't be so quick to write off the 2400G. Picking the 2400G gets you all cores on one CCX compared to 2500X having cores split by the Infinity Fabric interconnect. You also get a iGPU on die that might be useful if you plan on repurposing the APU for another role, and in case of card failure or troubleshooting.?
The 2500X on the hand has improved latency at the memory controller, L3 cache, and L2 cache levels.
Agree with you on the 2500X strong points, its been shown over the years how lots of cache improves gaming performance. Numerous links from quick net search will show this.
I built a 2400G system for a relative a month ago (my first Ryzen build) and I found it to be quite frankly, amazing. It'll run GTA4 and 5 using "onboard"! Gaming aside, it's a really great APU, IMHO. I couldn't tell the difference in general performance compared to my 4.8 6700K + RX580 rig, and I left the 2400G stock.
I spose the 2500X will have the improved turbo and will perform better than the 2400G, but that onboard is pretty sweet if you're not an elitist gamer with your displays.
I'm single monitor gamer only. Can't stand split screens. Will be using my R9 Nano for time being so 1K to 4K (VSR on 2 yr old screen) is where its at for me.
You might have painted yourself into a corner buying the RAM first. I always recommend selecting the motherboard and QVL listed CPU first, then refer to the motherboard's RAM QVL to ensure you get compatible RAM - RAM with the same specs as QVL listed RAM for that specific motherboard.
Not all DDR4 is supported by all DDR4 compatible motherboards or CPUs.
Know what your saying, 1st time doing a build with ram first. But if you knew the deal I got on this brand new ram, you would probably do the same thing. A perusal of the net anecdotally shows this particular ram is fantastic OCer even on Ryzen systems & DDR4 prices were a stumbling block for my budget, at least OC friendly pieces anyway. I can not justify upgrading due to those relatively high DDR4 prices for new gear otherwise.
This is why I wouldn't get a 2400G for a system that I knew was going to have a dedicated GPU (even though w1zzard's shown that lanes aren't all that important):
Agreed, that's how my logic works too with gaming cpu focus.
The 2600 is $10 more than the 2400G, I see no reason to even consider the 2400G unless you absolutely need the iGPU.
What part of the world are you in? cause' that's far from the situation where I am... unless I want to pay a premium in post/packing charges & that destroys the reason for buying 2600 for "only" $10 more.
It is just a different CPU for a different type of rig.
You never 'waste' cores or threads really, all you get with those 2 extra cores is being more future proof, ie the rig will last longer before its performance tanks.
2400G = ideal for a small form factor/silent and average performance rig that you won't push to limits and perhaps you may add a mid range GPU in due time.
You're only deluding yourself if you think 6c CPUs won't be useful for gaming. They already are. But it only shows at the higher end of the performance scale, so with fast GPUs. Even SMT, while being better than HT, won't benefit gaming in a consistent way. Only physical cores can guarantee you that and games really are closing in on the capabilities of the best CPUs faster than the CPUs gain performance. That has been a trend for years now, and this core count increase is long overdue really.
I built a 2200G ITX system for my parents a month ago. It works beautifully, but I would never pair that with powerful GPUs and call it a full fat gaming rig. It simply isn't. It lacks clockspeed to push high FPS and it lacks the cores to run anything on the background while gaming. Its also a total waste of the IGP to go that route with such a CPU.
But if you must choose between 2400G and 2500X... I would say 2400G because you get more bang for buck. You could use the CPU in any use case as an allround solution, the 2500X will require dedicated GPU to go along with it. The performance gap between these two quadcores is too small to justify losing that advantage I think. Both will perform largely the same in most games if your FPS target is 60.
If your choice would be between 2400G and 2600, now thát would really be a meaningful choice and thén I would suggest to go the 6c/12t route.
The upgrading for "future proofing" argument is long in the tooth imo.. That was my thinking back when I went all out with my budget & got i7-860 back in 09'. The potential to have 8 logical cores in windows 7 was a good idea at the time "in case" game devs decide to exploit all those cores... but this never happened, at least for games I was interested in. So that old i7-860 had its HT disabled cause'
it was useless for yrs until around 2015 or so. Besides that, OC it with HT enabled made that 45nm fab cpu hot pretty quickly, even with high end air cooling in full tower case.
When 7nm Ryzens released 2nd half 19', a re-evaluation of the scene will become a necessity imo. But no one knows for sure what cpu models will unravel then, but I'll bet they will be compatible with current AM4 chipsets if AMD history is anything to go by. But for now, start get my game rig ready for another hot Aussie summer coming our way & power usage is an issue so I'm keen on low TDP cpus. 95w TDP with 2600X is an absolute ceiling imo, but if I can avoid it I will
Budget wise, its' not justifiable with 2600X, nearly Au$100 diff between 2600X & 2400G atm, at least from reputable online Australian dealers.
Seeing that the vast majority of PC games even today still use "around" 4 or up to 8 threads/cores makes the argument for quad cores with SMT or HT more relevant. 
