OptiPlex 3020 SFF max RAM is 2x8GB can insert 8GB+4GB=12GB

Have you read the Memory guidelines on page 19 in the manual?

topics-cdn.dell.com/.../optiplex-3020-desktop_owner's manual2_en-us.pdf

You can mix module sizes but it doesn't say anything about ordering them according to GB size. There are rules for ordering modules of different "rank".

Keep in mind that both modules will run at the speed of the slowest module, if their speeds are different.  

And it doesn't say if they'll run as dual channel RAM when they're not exactly matched modules.

First socket Ram is nearest socket to processor??

By saying RAM chips must be same configuration but capacity may vary,  what means same configuration??

Don't worry about 'configuration' for the moment.

You'll have to look at the motherboard to see how they're numbered because the manual doesn't say. The slot numbers should be printed on the board right next to each slot.

Have you considered getting 2 matched 8GB modules to max out the system? Yes, I know that costs more money, but then you won't have to bother with order, rank and configuration...

And if you get them from a place like crucial.com, they guarantee the RAM will work if they recommended the modules for a specific system or they'll take them back.

label is 1/5 distance from the end (not the end near chassis)

2-3cm from end

I opened my Dell

slot nearest processor labelled 1 and other nearest hdd labelled 2

this is an old system ... if you was in my place you will got 2x8GB and sell 2x4GB in actions online?

Don't know what you mean about the label distance. The slot numbers should be printed directly on the motherboard and not on some label.

Even if you install them in the "wrong" slots, it shouldn't damage anything. At worst, it might not boot or maybe not recognize the total RAM installed, but then you can always switch them around in the slots.

Only you can decide how much money to invest in this system. If it's in good condition, does everything you want and you're going to keep it for a while then 2x8GB might be a reasonable investment.

It also depends on what you do with this system. If you're just surfing the net and reading email, probably not worthwhile to go to 16 GB. But if you watch vids, play games, run lots of apps at the same time, or apps like Photoshop, then max RAM might be good.

Have you checked online to see how much you might get for the 2x4GB pair? That might give you an idea how much 2x8GB will end up costing after you sell the 2x4GB...

Don't know if it makes any difference as long as both slots have a RAM module installed.

Obviously, if you only have 1 module installed, it should go in slot 1.