Since you're not using a Tablet PC you won't get the native Ink-based
handwriting recognition that you would get with a Tablet.
The Pegasus should work as any other external digitizing tablet or pen would
- it is treated as a mouse by OneNote; you can "write" using it, but anything
you write would be treated only as a drawing. OneNote will not convert the
drawing to text or allow you to search the contents of that drawing.
Having said that, I'm not sure if the models perform their own handwriting
conversion (or really how that specific products works). The website
mentions that it is compatible with the Microsoft Handwriting Recognition
Engine, which is separate from the Tablet/Ink engine.
Given that, I might recommend using the Pegasus with either its own built-in
recognition (if such a feature exists - io just don't know) for data entry
and then use OneNote to organize the notes you take.