You're gonna love PVD. Modern, clean, open, fun, great views, great planes. The airport's two major carriers couldn't be any farther apart, in walking distance. USAirways occupies six gates in the terminal's south airside. Southwest utilizes four gates at the northernmost end. The terminal is a two-story structure, with a "T" configuration, with ticketing and baggage check centrally located. DL, NW, CO, AA, UA, Air Ontario and Cape Air all have either one or two gates each.
You can take a cab from PVD to the downtown Amtrak train station in Providence. 15-minute trip. That line connects with Amtrak and the "T" (MBTA) with direct links to Boston, and all parts in between. Bonanza Bus is also good. Hotel buses are do the trick. I believe MBTA has a web site with schedules. Have fun.