Train tickets that are booked in advance are usually made up of two parts: the ticket itself and a seat reservation. A railpass will cover the 'ticket' part of all your journeys, but does not cover any reservations.
Most regional trains do not require reservations, and so you can travel on these with just the railpass. But if you are planning to travel long distances, it's likely that you will always be travelling on trains where reservations are compulsory.
http://reiseauskunft.bahn.de/bin/query.exe/en is an English version of the bahn.de timetable search tool. You can use this to see if your trains require reservations:
However you should be careful when using bahn.de because the data is not always fully up-to-date for trains outside of Germany.
I'm not 100% sure, but I don't think it is possible to purchase reservations directly from http://voyages-sncf.com. This is because that site is designed for French people, and the railpass rules are that you can't use a pass for the country you are resident in. You can definitely purchase the reservations from any one of these SNCF subsidiary websites:
Note that the prices on these different websites will not necessarily be the same, because each site sets its own exchange rates. SNCF has a few other subsidiary websites, including http://tgv-europe.com, but I don't think it's possible to book reservations on these sites.
You can also purchase reservations from Loco2.com, which I run:
Our site has a connection to the Rail Europe booking system. The prices for reservations on our site will be the same as at raileurope.co.uk (but we are a completely independent company, and we take international credit cards).
Before you actually book the railpass or any reservations, you should consider the following:
Hopefully it will soon be unnecessary to do so much manual research to work out the costs/booking options, but this will require the rail companies to be more open with their data (for example so that the timetable data accessible via bahn.de can be combined with the booking functionality for actually purchasing the reservations). See http://www.wired.co.uk/news/archive/2011-06/15/european-rail-data