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Newegg.com has the Western Digital Caviar SE16 WD6400AAKS 640GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive - OEM for $79.99 with free shipping.
Use $5 off promo code EMCAJCGBK (Newegg Newsletter Subscription - Sign up at this link )

Other deals:
* Seagate Barracuda 7200.10 500GB SATA Hard Drive $69.99 shipped
* Western Digital 640GB SATA Hard Drive $84.99 shipped
* Samsung 1TB SATA Hard Drive $149.49 shipped
Final Price for this deal: $84.99 - $5 = $79.99 + Free shipping