We are flying to Madrid - should have chosen Barcelona. How difficult would it be to land at the international airport in Madrid and fly right to Barcelona?

About as difficult as in any other hub airport & subject to the same possible things like immigration, collecting luggage, customs, changing terminal, checkin, security...

The website for Spanish airports is www.aena.es just pick Madrid Barajas

why not give madrid a try? there's loads to see and do, and its a much less aggressively touristed town than barca. you can then easily (and relatively cheaply) fly on to barcelona via vueling (www.vueling.com) or iberia

There are many flights from Madrid to Barcelona. Just be aware that Terminal 4 is onlu sed for Iberia flights so if you fly from the states in another airline and then want to fly Iberia to barcelona you ahve to change terminasl ( Terminal 4 is not connected to terminals 1 through 3).
Other airlines between Madrid and Barcelona are Spanair and Vueling

Gon on the net and book the flights NOW if you are meaning to do this; the prices will get higher the longer you wait.

Since those three airlines compete heavily on that route you can often get even one way fares relatively cheap.