Yeah--I figured out that the x1 3/8 were the wrong size before I bought them--as you mentioned, the BSD for fractional 26" tires is different from decimal 26". I needed the 559s; I'm gonna go with some 26x1.5 Continental Cross Country, which looks like a safe bet since I see lots of people here have liked them.
After a race or two, I'll figure out what else I might want to get. Just saw a Bianchi San Jose listed on Craigslist here in MA--SS cyclocross looks like it could be a lot of fun and the bike doesn't look half bad either.