Conflicting information in the "about this mac" window

Most likely because the first item is the result of a power on self test performed before macOS started, while the memory details were gathered once macOS was running and that did not trigger any memory issue. The memory error occurred during POST where a self test was performed....the selftest was not likely performed with the latter. Keep in mind that memory issues can be intermittent & may only be detected under certain conditions unless the memory failure is severe enough to persist all the time.

You can try running the Apple Hardware Test/Apple Diagnostics to see if it can re-detect the memory failure. How you do this depends on the exact model Mac since Apple had the diagnostics on the original DVDs with the older Macs while some later Macs had online diagnostics added later on. Your Mac is old judging by the 800MHz bus speeds.

One of your memory modules or memory slots has a hardware issue. You can try creating a bootable Memtest86 USB stick to check the memory, but keep in mind it may not always detected an issue.

FYI, a passing diagnostic does not mean everything is good. It only means that the tests at that specific time did not encounter an issue.

You will need to boot from the second DVD which shipped with the Mac from the factory as it contains the Apple Hardware Test.

Then you should take it to a local Apple expert for assistance if you feel it is worthwhile to spend money on such an old system. Apple won't help you here. Hard to say if an Apple Authorized Service Provider will assist you.

Or it may be time time to purchase a new computer

Apple disabled the built-in testing in a number of older models - not sure if this is entirely accurate, but I think it boils down to, if your Mac is new enough (post 2013/14) to use Apple Diagnostics, it still works, but if it is old enough that it uses Apple Hardware Test (AHT), the D key on startup is no longer working.

What about using the "C" key.

Not sure why the "D" key did not work for the diagnostic.

You can try making a bootable diagnostic USB by using the information here:

https://github.com/upekkha/AppleHardwareTest

All of the download links from that site actually download directly from Apple's own servers.

Have you tried creating & using a bootable MemTest86 USB stick?