We recommend Seagate because of their five year warranty. A Seagate 80 GB drive is available with free shipping at www.zipzoomfly.com for $48.99 and a Western Digital is $47.90 among others. But a 160 GB drive is only $58.99 and a 320 GB drive is merely $76.49.
You may have to update your BIOS by download from www.hp.com entering your model number.
You will find similar prices at www.cdw.com, www.newegg.com, www.techdepot.com, www.outpost.com, www.directron.com and so on.