I'm no extended expert (and that's an understatement) but from what I understand, viable archetypes include:
- Tog
- Affinity
- Aluren
- Goblins
- Red Deck Wins
- Life
- The Rock
That's about as varied as formats come. Therefor, it'll be difficult to SB correctly if you don't know the people who will be playing and what their preferences are. If you check the WotC pages, you can find the t8 lists for every GP that's been held so far, which should give you a general idea of what's good.
http://www.wizards.com/default.asp?x=sideboard/coverage#2005Click on the event you want to see the results of, and scroll down the coverage blog to find a link to the T8 lists. Example (GP Eindhoven):
http://www.wizards.com/default.asp?x=mtgevent/gpeind05/welcome#27Michael Leicht placed 6th there with this, so it can't be
too bad...
9 Island
6 Forest
4 Yavimaya Coast
4 Basking Rootwalla
4 Wild Mongrel
4 Aquamoeba
4 Arrogant Wurm
2 Wonder
4 Careful Study
4 Chrome Mox
4 Circular Logic
4 Daze
4 Deep Analysis
2 Intuition
1 Roar of the Wurm
SB
3 Oxidize
4 Chill
4 Seal of Removal
3 Stifle
1 Ray of Revelation