I return with answers!!
This info is from an European 'groundhopper'. So should know more than the simple football fan.
Personally I doubt it will sell out. Buying Barca tickets is easy:
1. go through one of the many agencies on the internet, they should send the tickets to your hotel (they won't post them to you)
2. go to any branch of the Caixa bank, put your card in the ATM and select the "Buy Tickets" option. You can then purchase your Barcelona ticket and it will be printed out on the spot. I'm not sure if this works for CL games though
3. go the main city tourist office on Placa Catalunya, they sell tickets, though perhaps not for CL games
4. go the stadium on the day of the game (or 1 or 2 days before), ideally in the afternoon, then find the ticket office. Even if that isn't open then there are always lots of old Spanish guys waiting around to sell tickets. Of course they want a bit extra but they don't charge silly prices.