D800 and Insprion 8600 is very similier.. but not the same... that's why Dell need 2 different lines.

D800 is design for business in mind... so the overall build quality is a bit better, it has alloy case and GB ethernet card.

Insprion 8600 is for home user. it can change case color. but the case is plastic.. and only 10/100 ethernet card.

for the same future... D800 is usually 2-300 USD more expensive.