Do you know what size bike you are? If you are looking at used in the Chicago area, when I was glancing around yesterday I came across http://chicago.craigslist.org/chc/bik/2263270514.html . I'm not sure what the cassette is on it, but other than that it looks like it may be a throw a rack on and ride bike. Theres a couple of nice upgrades like the marathon tires (if they are in good condition) and the Nitto Noodle bars. It's double the price of the FX though, and you didn't mention your price range on bikes.

I also know where (as of a week ago) there was a NEW 59cm 2008 Jamis Aurora in Chicago for $641, which is also a good deal, but again not sure on your size. A little bit different geometry on both the Volpe and the Aurora than a "real" touring bike (arguably), but closer than the FX. I personally prefer the geometry to that of the other touring bikes I tried as the short top tubes make life easy on my back, but some people like the stretched out feel.

I'd say for a trip like you are planning, first thing is to make sure you are good and comfy on the bike. An annoying wrist/back/whatever feeling riding around the block will only get worse going across the country. Then make sure the bike has the strength and storage space for everything you want to bring with.