OptiPlex GX620, what OS would would run on this?

On a very old, almost 17 years old, system I was curious to search the Internet for answers. Surprisingly Newegg has this exact system for sale running Windows 10 with similar specs as your system. Don't expect decent performance as this is really pushing the system hardware.

Alternatively Windows XP was the original XP on this system and possibly Windows 7. But above XP the OS might be a bit of an adventure.

In agreement with JOcean.  Community member Speedstep regularly mentioned running Win10 on his GX620.  I've seen it run on a 380, but very herky-jerky just for the internet.

I take it you really want to keep the 620 going.  I suggest 4GB RAM (max per spec.).  Win10 won't run the fastest on 4GB like it does on 8GB, but hopefully isn't choppy with 4GB RAM.

I certainly don't recommend Win7.  Not just because it's EOL, but because Win7 DVD's have the initial version which was too raw to do much of anything with and it lacked functionality.  And of course, no more updates (new or old) from Microsoft.  If you did have working Win7, you'd need to use your own security software.

WinXP ran good right out of the box.  Not in use, but I still have the CD and Service Packs 2 & 3 on CD.  You'd need your own security software if going online with it.  Either way, one should never be without security software.  Too easy to sneak malware with a photo of a family member, which happened to me in 2013.

Also keep in mind Google support for Chrome on Win7 ended on Jan. 15, 2023.

I tried to bring it up to 4gb and it just wouldn't post

I give up I can live with Linux and abit of lag

Sorry to see it didn't work.

Did the 4th stick of RAM match the first 3 in speed (Mhz) and volts?  We already know the size is the same.  Some Dell's and other PC's are picky even if brands are mixed.

It means I can't read a all text not messages and other things

I can't read so voltage I'm not gonna be able to find this I was mixing Samsung 533mhz sticks with Acer ones it didn't beep at me but the bios freaked out and tne screen had abunch of garbage on it

Uhh kubuntu came on this and kubuntu on 3gb of ram is asking for trouble

I'm lucky it turns on at all

All my sticks are 533mhz

On a working PC, you will be downloading Win10 to an 8GB or bigger blank flash drive formatted to FAT32.  They often come formatted that way anyway.   Download Windows 10 Disc Image (ISO File) Microsoft site    Download the ISO.  It will be in your Downloads folder.  With flash drive inserted, click on the ISO and follow onscreen instructions   Win10 will then be loaded on the flash drive.  Takes a little while.  When ready to load Win10 from flash drive, boot from from flash drive on your PC to load Win10.

When a window pops up asking what partition to load Win10 on, delete all previous partition if reusing a boot drive.  Then click on Next or Install.  Win10 will then automatically create the appropriate partitions and load.  When a window pops up asking for product key, click on link that says "I do not have a product key."  Win10 will then continue to install using the product key that's imbedded in BIOS.  You'll still need to activate it online.

Edit: The above is copy and pasted.  With a GX620 and Legacy BIOS, I can't guarantee you won't need a product key, but you can try it.

If you don't have a working PC to download Win10 on, you'll have to buy a Win10 DVD.  I'd link the one Newegg had for $14.99, but now they don't have it.  But here's one example on eBay:  Microsoft Genuine Authentic Windows 10 PRO + License DVD 

Win10's minimum requirement is 2GB RAM.  It'll still run on your GX620.  Below is a screengrab of the system requirements:

Tap or click on image and it'll show clearer.

I have a Windows 7 laptop and the drive is from a laptop

Now that I finally got to load Win10 from a flash drive for somebody else, I made a revision to the 1st paragraph in my post above.

As for your post above, not sure if you still had a question?