Get It Now From Newegg
$109 is the lowest price so far for a 640GB SATA drive and if you held off until now on buying a new hard drive, you're probably not going to get a much better deal until next year, when 1TB drives are expected to sell for this much.

The WD6400AAKS model is getting some great reviews over at Newegg and it comes with a 3 year warranty. You can buy it for $109.99 with free shipping.