Actually if you configure them the same, (just did it) the Dell is $1583, on sale for $1153 while the Mac is $1224 every day low price. So normally the Mac is $360 less and only $70 more when the Dell is on sale.
Add in the additional SW for virus/spyware protection and it's easy to see how much better a deal the Dell is
The only way the Mac is $400 more is if you bought a Dell box that had a lower system spec than the base Mac. They were not equal spec boxes.
