If you go full frame there are several lenses that won't work, they are consumer lenses and manufactured for the bodies that consumers would purchase with a mind to keeping them affordable while giving good quality. They are the DX lenses-which is your 55-200 and the 18-55 that you probably have.
If you are thinking about switching camera brands it's an option to sell everything you have and invest in something else, but you are familiar with Nikon. If you are going full frame? you aren't married to Nikon at all because your lenses won't work on Nikon. If you are seriously considering the D3s? I'd probably go with that over Canon because it's what you know. You're talking $5200 camera only and all new lenses. If you are considering the D3 you're looking at used about $3500 and all new lenses.
If you are thinking the D300s you can use your lenses and we're talking an investment around $1700 in the camera. You would have to ask the Nikon shooters which they'd go with between the D7000 and D300s because I am not that familiar with the D300s to compare it.
If you were really considering the D3X I'd switch to the Canon 1DX. It's at about $8000 body for the D3X and I believe Canon announced the 1DX coming in the same price range with an IMPRESSIVE set of new features. If you're dropping 10 grand? buy the best on the market and as of today, the 1DX is it.
Jumping from a D3100 to a D3? That's huge. There is no auto when you get up there. There are priority modes, but you need to know what you are doing too. Honestly? I'd be looking at the D7000 or the D300s. We have also skipped the D700 which falls in at $2500 or somewhere around there and means new lenses.
MUCH of what you aren't seeing is coming from things you probably still need to learn, however you would actually SEE an improvement by significant jump just from to the D5100.