this is current rig. I am posting it here so I have it.

$ sudo dmidecode --type 17
# dmidecode 3.6
Getting SMBIOS data from sysfs.
SMBIOS 2.8 present.

Handle 0x0043, DMI type 17, 40 bytes
Memory Device
	Array Handle: 0x0042
	Error Information Handle: Not Provided
	Total Width: 64 bits
	Data Width: 64 bits
	Size: 8 GB
	Form Factor: DIMM
	Set: None
	Locator: ChannelA-DIMM0
	Bank Locator: BANK 0
	Type: DDR3
	Type Detail: Synchronous
	Speed: 1600 MT/s
	Manufacturer: Samsung
	Serial Number: 17021E38
	Asset Tag: 9876543210
	Part Number: M378B1G73EB0-CK0  
	Rank: 2
	Configured Memory Speed: 1600 MT/s
	Minimum Voltage: 1.5 V
	Maximum Voltage: 1.5 V
	Configured Voltage: 1.5 V

Handle 0x0044, DMI type 17, 40 bytes
Memory Device
	Array Handle: 0x0042
	Error Information Handle: Not Provided
	Total Width: 64 bits
	Data Width: 64 bits
	Size: 4 GB
	Form Factor: DIMM
	Set: None
	Locator: ChannelA-DIMM1
	Bank Locator: BANK 1
	Type: DDR3
	Type Detail: Synchronous
	Speed: 1600 MT/s
	Manufacturer: 1315
	Serial Number: 00000000
	Asset Tag: 9876543210
	Part Number: CT51264BA160B.C16F
	Rank: 2
	Configured Memory Speed: 1600 MT/s
	Minimum Voltage: 1.5 V
	Maximum Voltage: 1.5 V
	Configured Voltage: 1.5 V

Handle 0x0045, DMI type 17, 40 bytes
Memory Device
	Array Handle: 0x0042
	Error Information Handle: Not Provided
	Total Width: Unknown
	Data Width: Unknown
	Size: No Module Installed
	Form Factor: DIMM
	Set: None
	Locator: ChannelB-DIMM0
	Bank Locator: BANK 2
	Type: Unknown
	Type Detail: None

Handle 0x0046, DMI type 17, 40 bytes
Memory Device
	Array Handle: 0x0042
	Error Information Handle: Not Provided
	Total Width: 64 bits
	Data Width: 64 bits
	Size: 4 GB
	Form Factor: DIMM
	Set: None
	Locator: ChannelB-DIMM1
	Bank Locator: BANK 3
	Type: DDR3
	Type Detail: Synchronous
	Speed: 1600 MT/s
	Manufacturer: 1315
	Serial Number: 00000000
	Asset Tag: 9876543210
	Part Number: CT51264BA160B.C16F
	Rank: 2
	Configured Memory Speed: 1600 MT/s
	Minimum Voltage: 1.5 V
	Maximum Voltage: 1.5 V
	Configured Voltage: 1.5 V

right now I have to move my monitor, power cords, ethernet cord, and keyboard just to use the new rig. so that will be tomorrow.

@Axios under 'configure memory speed' is what is 1600MTS? and is that fast or slow? may not stay up so talk later. and thanks.

    Axios they make a "tiny" and a "tower" and the SFF--I got the SFF. It's got the usual Lenovo swinging gate drive bay with the cords tucked underneath and barely adequate hand room like under the hood of a typical 4 cyl car.

    Stuff I'm reading says 2666 MTS is good. I have 1660. While the industry says MTS is more accurate then MHZ it also depends on bus/mobo.

    So I'm as stupid now as before my 1 hr of reading.
    All I learned in 1 hr was the highest number is better. But in relation to what? Bus? Slots? I usually get more substance for my hour researching must have been an off night.

    The only question now is how would I know that 24GB was worse than 16gb? Crashing now. Thanks for the info axios.

      brent So I will put in my 8 and keep three of the 4's. (24GB will be the most I ever had!)

      20GB, not 24.
      Is it worth it? It's all the same speed, so likely wont harm anything. Conventional wisdom is you want your ram in matched pairs so it is always double data rate (DDR). I think if you pair the 8 with a 4 in the first set of slots, you'll get 8GB DDR, then 4GB SDR (half speed), before the last pair at 8GB DDR. This will be slower when you are using more than 8GB
      tldr: match 2X4GB in the first set of slots & 1X4 with 1X8GB in the second set of slots.

      Bonus round - you get to figure out which slots are 1st & which are 2nd

          I am a Techi always look in the manual..lol

            murbert Conventional wisdom is you want your ram in matched pairs so it is always double data rate (DDR).

            So my one 8 and two 4's were always 8? Or 12GB?
            Also thank you for the slot info-will let you know when I get to the Bonus Round. 🙂

                Axios I read it twice i did not see the part about bus speed/mhz/memory, but I do skim sometimes.

                    brent 12, but 4 gb would operate at SDR. That'll be a noticeable slowdown.

                        murbert 12, but 4 gb would operate at SDR. That'll be a noticeable slowdown.

                        that's fascinating I never knew that. my goal for 20 GB would be futile then with 3X4GB and a single 8. Would come out 4GB better on the new one that imaginary 16GB (12) on this one. My eyeballs hurt.

                            brent
                            that's OK. I'll tell you a weird story. On my new SSD I actually cloned Solus and Endeavour (backup). Only Solus booted.
                            I had 4.6 but I said xxxx it because I never cloned before and 4.5 with an update is allegedly 4.6. Anyway I did find a Lenovo Thinkcentre big (25 page) pdf all about Linux and how to partition for it etc etc. That will be my after dinner reading.

                            @murbert @Axios that's for your input gents I need it. I'm doing new things.

                              craigtoyoracer you can pick up DDR3 2x8GB cheap like $30-40. Looked on Newegg for you. 🙂

                              Matched new ram is worth it.

                              what is it about hardware that when it rain$ it pours. I was donated two 2GB laptops and getting them to 4 is more memory!
                              --What's the rule of thumb for matching old/exisitng 8GB with one new 8GB I wonder?

                                  brent I would say matching new/old last resort.If you do make sure the numbers are the same speed and timing, is very important.

                                  When upgrading , rule get as much ram as you can realistically. Ram and SSD are the best bang for the buck on older equipment.

                                      craigtoyoracer
                                      the underlying message I'm taking away from this thread is don't take shortcuts with Hardware 🙂.