Bob,
With regard to Round-tripping, you have 2 options:
Send to STP Audio File Project
Send to STP Multitrack Project
If you select a single clip in Final Cut,
Send to Audio File Project opens the file in the STP file editor. Your edits will update dynamically (upon saving) in Final Cut.
If you send the *entire* sequence out as a Multitrack Project, you need to save your changes in STP, Export the project, and prior to doing so - select the "Send Files to Final Cut Pro Sequence", located in the "After Export" pop up in the Export dialog.
That's about it.
As far as noise reduction, if you are not happy with the results in STP, I suggest looking into something like MR Noise by Wave Arts, or RX by iZotope.
http://wavearts.com/products/plugins/mr-noise/
http://www.izotope.com/products/audio/rx/
-paul.
