Old thread I had created on this, I'll just cut and paste.
Microsoft Mid/Late 2007 Price Guide
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$399 XBox 360 Premium 80GB New SKU (Slight redesign, Internal PSU, HDMI support, Quieter .65u XCPU)
$349 XBox 360 Premium 20GB Old SKU
$299 XBox 360 Core New SKU (Slight redesign, Internal PSU, HDMI support, Quieter .65u XCPU)
$249 XBox 360 Core Old SKU
$149 HD-DVD Addon Drive
$99 80GB Hard Drive
$69 20GB Hard Drive
$39 256MB Memory Card
Microsoft Mid/Late 2007 Price Guide
----------------------------------
$399 XBox 360 Premium 80GB New SKU (Slight redesign, Internal PSU, HDMI support, Quieter .65u XCPU)
$349 XBox 360 Premium 20GB Old SKU
$299 XBox 360 Core New SKU (Slight redesign, Internal PSU, HDMI support, Quieter .65u XCPU)
$249 XBox 360 Core Old SKU
$149 HD-DVD Addon Drive
$99 80GB Hard Drive
$69 20GB Hard Drive
$39 256MB Memory Card