Well, for under $500 you could get a Dell with a x2 3800+ proc, 1g ddr2 533(budget ram but could be worse) dvd-rw, 160g hdd and some random budget gfx card.
$175 for an x2 3800 with a fan...
$140 for 1g ddr2 533 value ram from most companies...
$40 for dvd-rw drive...
$55-65 for 160gp hdd...
$410-420 in parts so far...So the rest of the $80-90 goes into the psu, mobo, case, and 7300LE low end budget card($50 card).
Doesn't look like double the price....Although custom computers are usually better then one made by a manufacturer including Dell.