I have to present a prototype of an web app in the following days to one of my clients, the thing is I\'m not so good at CSS and worst of all I\'m almost never happy with the re
wireframe mockups like that are a brilliant way to start.
Having used most of the UI framewroks discussed here, I'd liek to steer you towards jQueryUi for the following reasons:
jQueryUI CSS framework takes care of the consistent and cool looking CSS for you (it's really easy - just make some markup and apply the classes)
jQueryUI has tabcontrol, and heaps on neat quick easy ways to style forms.