First of all ... ignore the fps. With ToUcams, most of the time you'll work with 5 or 10 fps. That's if you're imaging planets. Anything frame rates greater than that compress the data too much.
I have the 840K and it's a great little lunar and planetary webcam. In it's modified form (to allow longer exposures) it's also capable of quite decent DSO captures.
I've not used or compared the 900NC but some of the other guys have them and swear by them. It uses the same Sony CCD 1/4 inch chip as the later 840K Pro II's, but has slightly greater resolution due to a few more pixels (I think?).
Mike's DMK is a significant step up from a ToUcam in terms of $$$ and capability. It's also a monochrome camera which is an equal step up on the learning curve if you want to use red, green and blue filters with it to capture colour images.
Keep asking questions
