Buffy is dead, but...
The show is not (there are actually 2 more seasons confirmed, so don't worry)
Buffy will still be the main character.
Giles will only be a recurring character in series 6 because Anthony Stewart Head wants to spend more time in England with his family. Apart from that, the cast list wont change (Tara will still be recurring).
Sarah Michelle Gellar will still be playing the Slayer.
Sarah Michelle Gellar (as Buffy) will be in the first episode of the series.
She will be the same Buffy as we have been watching for the past 5 series (not an alternate universe version for example).
The first episode of series 6 will a double episode
There are no plans to do any crossovers with Angel next season, and because the two shows are on rival networks as of next season, it is very unlikely that any will happen at all.
Episode 6 will be a musical episode with all the songs written by Joss Whedon, who will also direct the episode. The music will be more like pop music than what you get in the West End.
Jonathan will appear in al least one of the first few episodes.
The theme for the sixth series will be 'Oh, Grow Up' (the fifth series' theme was 'death') and will see the Scoobys making mature decisions that are not always correct and learning why grown ups act the way they do.
Joss Whedon will share the title of Executive Producer with Marti Noxon this season, though Joss still has the final say.
Amy Madison, the fellow witch seen in the episodes 'Witch' and 'Gingerbread' who is currently a rat, will be de-ratted this season.
Joss would like to get Seth Green (Oz), Eliza Dushku (Faith) and Marc Blucas (Riley) back on the show, though as they are all now persuing successful movie careers, that might be difficult.