You need to run the Dell diagnostics outside of WIndows - NOT under Windows. It WILL generate a numeric code for any fault that shows up.
The error you've posted suggests a problem with the L2 cache on the CPU -- which in turn could be a bad CPU -- or it could be a bad mainboard. The latter is far more likely than the former.
This could be a bad CPU, mainboard or if the system has a separate video card, bad video card.
What are the symptoms of the problem (i.e., what's not working)?
Complete, working 6400 systems look to be selling for between $30-50 on EBay - I wouldn't spend much time on replacement parts -- it'll be cheaper to simply replace the system than to swap parts into something this old.