I have an HP Pavilion ZV6000 with a crashed HD. To order a new HD, what do I need to know? The old one is 40 GB, PATA, 2.5" and 4200 rpm. I would think the size can vary and I know the physical size cannot. So, what about the rpm and PATA part?

what do I need to know? The old one is 40 GB, PATA, 2.5" and 4200 rpm.
You have the necessary information. The capacity (40 GB) can vary, the interface PATA is generally constrained as well as the physical size of 2.5". The RPMs can vary with higher RPM drives having higher performance, however they also tend to generate more heat that has to be dissipated which can be problematic if the laptop doesn't have sufficient thermal margins and they can use more power which decreases battery life.

Resources:
Maintenance and Service Guide - HP Pavilion zv6000 Notebook PC, Compaq Presario R4000 Notebook PC (http://h10032.www1.hp.com/ctg/Manual/c00364919.pdf)
HP Notebook Hard Drive - Measures of care, diagnosis and maintenance (http://h10032.www1.hp.com/ctg/Manual/c00133452.pdf)
Hardware and Software Guide - HP Notebook Series (http://h10032.www1.hp.com/ctg/Manual/c00363776.pdf)

Thanks for the info!