Tiger Direct has a Seagate 320GB SATA-300 hard drive for $100, $80 if you don’t mind dealing with rebates. SATA-300 is one of several designations for the 3.0Gbit/sec spec for SATA drives — fast, fast, fast! And the Seagate comes with a 5 year warranty, too.
The rebate’s only good till 1/31. It looks like $100 is their regular price.