Its gonna be damn pricey. Me and my buddy spent like 50€ a piece! from Paris to Madrid last week. I know you have a railpass....but consider the bus. I heard it takes longer but much much cheaper.
As for booking online...i have no idea. I dunno wher eu staying, but Austelitz is not too far if you take the metro... esp if your hostel is centrally located. Or, you can just reserve it at the station you get there from. Hope it helps. And genarlly its cheaper to book in person.
Vincent: "So what you gonna do?"
Jules: "Well, basically, I'm just gonna walk the earth."
Vincent: "What you mean 'walk the earth'?"
Jules: "You know, like Kane in 'Kung Fu'...go places...meet people...get in adventures."
Trips (only counting recreational travel):
FIRST TRIP (2005): FIRST EUROTRIP EVER! UK, France, Spain, Italy, Germany, Holland
SECOND TRIP (2007): First Solo Trip! Greece, Turkey, Syria, Spain
2008: China (Beijing, Shanghai, Yangshuo) ...right before the Olympics!
2009: Japan & HK, Southern Spain
[size=1]2010: All over Lebanon, Ibiza (Spain), Oktoberfest (Germany), Thailand.
2011: India (Goa), Jordan, Jerusalem, San Sebastian (Spain), Amsterdam (again), London, Driving from Vancouver to L.A. (stopping in Portland, Seattle, San Fran and all the little stops), Montpellier (France), Geneva and Lausanne (Switzerland)
"Bite off more than you can chew, then chew it."