PC Upgrading Help

Computers work as a single unit; no one thing has the "most" importance.
Also I highly doubt you use anything that actually uses 16 GB of system ram.... however almost all modern games likely can use at least 4 GB of video ram. So pay more attention to the amount of video ram you purchase. 8 GB of system ram is likely more than enough. Most modern games I've only seen hit about 1.2 GB of system ram. (That said... don't be scared to get 16 GB system ram... just be aware of what you're paying for.)
I doubt any of the games out there right now come anywhere near putting a full load on all the cores of even a quad core; remember having more cores doesn't mean faster... it means more work done.
If the software isn't designed to take advantage of the # of cores you won't see a difference. Example is Splinter Cell: Chaos Theory: it will run faster on a single 3 GHz core than it does on a multi-core CPU of only 2.1 GHz.
In other words... depending on what you're doing you might be better off getting an older i3 that runs at 4.1 GHz vs the newer CPU only running at 3.5 GHz. If you like older games they'll probably run faster for a higher cycles per second than more cores.
I currently run a 780 GTX for video myself & it has no problems pushing the frames per second... what I'm finding, is that at only 3 GB of video ram, it's starting to get limited on things such as high resolution textures, anti-aliased shadows & other "lots of little stuff" things that require memory. Speed isn't an issue though.

But the most honest advice I can give for buying anything is this: buy the best you can honestly afford. Don't buy for how "cool " it is or how much you like it. Buy it for reliability, support and feature set. Like the camera I sold to a teenager once... she bought it because it was pink. NO. BAD CUSTOMER. For the same price she could have had a bigger lens & a fraction more of a megapixel.
Don't go above your budget as it'll just come back to bite you in the ass later if you have problems with the item(s) but don't cheap out either... cheaping out just means you'll have to upgrade sooner or may not be happy with your purchase.

It depends how far you want to go and what you want to use it for.

If you want to play Doom with ultra settings it will require a new MOBO, CPU, and GPU and while 8GB is good enough right now I would actually recommend 16 at this point. I have 8GB myself and when I play some games I hit 90% ram usage.

If you want, you can start to piecemeal it right away. If you first upgraded the video card, you can buy some life for your current set up. Your current setup is not bad, but the main difference is the CPU. The old Mobo cannot handle a similar CPU, so you;d have to get a new Mobo and CPU combo if you wanted to fully upgrade, which gets expensive. The good thing about a new video card is that you can simply just put it into the upgraded PC later but it will allow you to play games for now.

Good luck!

Your old mobo doesnt support AM3+ so you wont be able to put in an 8320.

You can still squeeze more out of both PC with Rx 570. I have phenom ii x4 945 and went from barely playable fallout4 with gtx 460 1gb to high settings modded with Rx 460 4gb. If you can afford to upgrade everything I'd suggest and ryzen 5 16 GB ram and Rx 570 4gb. Do not buy you with less than 4gb vram.

. Being that I'm cheap gamer I bout my phenom PC used for $92.50, 8gb ram for $30, and Rx 460 4 GB for $70 new. And I only play modded fallout 4 and Skyrim, lol, and yes both games use up to 3gb vram. I also have a PS4 and rent from gamefly.

just a comment on ram: if you want to game on and use the rigs for the next few years only 8gb ram is not enough.

i've had 8gb ram since 2008 and nothing ever managed to grab all of it...
...until i started playing fallout 4 with the hd texture pack. 8gb ram is definitely not enough for that. sooner or later the system starts swapping to disk and then the performance totally craps out.

luckily i'll get an i7 with 16gb soon and my trusty old Q8400 will retire.

The Phenom II 955 you have in that old machine is technically better at single threaded performance (which is usually the most valuable for gaming) than your FX-8320s.
"Upgrading" that is basically pointless if your goal is gaming. So I'd just leave the MOBO & CPU on that alone. As to the RAM, while I'd generally say go for it on the 16GB for most anyone, all of your systems are using DDR3 and the price on that has more than doubled in the last year because everything has been switching to DDR4 and DDR3 production is now almost non-existent. The minimal amount of performance gain you'd get from adding more RAM unless you're doing some really high end stuff just isn't going to be worth it considering that 8GB is now going to cost you something like $70 and a 16GB kit will cost $130.

What I'd do if I were you is spend the ~$220 on an AMD RX 480 8GB version which will give you a GPU roughly on par with the nvidia 1060 for way less, with tons of VRAM to run just about anything you want. Anything else in there should wait until you do a full rebuild, shooting for current gen setup.

Stick the 750Ti into Pc 1 and get a new GPU for PC2.
You don't need 16GB RAM. 8 GB is fine for normal gaming. The PCs are too slow for heavily modded games or HD Texture packs anyway.

So I can get those components and put them in the older PC. Thanks.

The best upgrade you can get for those is an SSD, 4-8 GB more RAM, and GTX 1050 for the 1st PC and GTX 1050Ti for the other 2 I would say.