I have been using Cumulus by Canto for quite sometime and it works great on both mac and pc. It supports many file types from PSD to tiff jpeg, even pdf and quarkxpress files. Not to mention mpeg, and quicktime.
You can also make stand alone cd libraries to give to people and they don't need the software...
It is a drag and drop your picture interface and you can make custom fields to suit your needs. I am using this for an inhouse design firm for a corporation for all of our photography. Your best bet is to try demo's of all you can. You'll know what you need.
www.canto.com
The basic version would suit any entry level needs.
You could also check
www.extensis.com they have software called Extensis Portfolio, which is similar to Cumulus.
Both companies are leaders in image management and either will definately be a wise choice, even if you don't use all the features.
Hope that helps.