Here is the Dell Dimension 5000 Specifications
If you notice, the available CPU combinations are not spelled out clearly. There is always a reason for this. Not all available CPU were tested at the time the board was specified.
The significant difference between the P4 5 series and 6 series CPU is cache, much like the difference between the P4D 8 series and 9 series.
The FSB on the 5000 Dimension can handle up to 800MHz according to the specification for the system clock.
The P4 6XX should work in your system as long as it is not an EE.
Many thanks for the answer from a bystander: I did not ask the question but I appreciated the excellent answer.
As I am still not very knowledgeable with these matters, I would appreciate some further clarification:
in the sentence "The P4 6XX should work in your system as long as it is not an EE. " what does the EE stand for or what's the difference?