You might also check out
ArgoUML. I haven't really used either Argo or
Visual Paradigm SDE, but Argo is open source and free.
I'll say it again though, and maybe a bit stronger this time: UML diagrams are not a way to figure anything out. Analysis and design techniques are a way to figure things out; UML is a tool. For example, the statement, "I'm a great programmer: I know language X," doesn't make much sense, does it? In the same way, "I'm a great designer: I know UML," shouldn't make much sense either.
If you don't know
how to use UML (and I don't just mean notation), I don't think a fancy tool like an Eclipse plug-in is a good place to start. Paper, and maybe a book on design using UML, would be.
"Owning a hammer does not make one an architect."