That series of desktops was discontinued in 2007 as the following statement indicates. "The Dell Dimension series was a line of home and business desktop computers manufactured by Dell. In 2007, the Dimension series was discontinued and replaced with the Dell Inspiron series for low-end models and the Dell Studio series for higher-end models." So posting in the Inspiron desktop forum was the correct step.
After logging in here, and trying to get some technical help with an older desktop that I have, I have noticed that the drop-down menu of categories does not include Dell Dimension devices anywhere. As such, I am not clear where I should post a question regarding a Dimension 8300 desktop that I have. As such, I have ended up posting in the Inspiron Desktops category like now. Where should you post a question about a Dell Dimension desktop computer here please, and Thanks in advance!!  
Why is there no Dimension Destops as a category?