Yes, we did had to invest on new servers. Not only we need more RAM, we also needed more storage (which as expensive because we use SSDs). All things considered, SQL Server licences are not cheap either (6K-7K each) so in the long run I believe we will be saving money.
Our training was very much hands on: (1) we migrated a piece of code, (2) tested against a full-sized database, (3) profiled performance under load, (4) learned from the results, moved on the next piece of code (1).
Our plan if MongoDB wasn't good enough consisted basically of: (1) crying ourselves to sleep, (2) drinking heavily, and (3) contributing to MongoDB's development to make it good enough.
Regarding requirements, we were looking for something that was free and open source because we are planning to scale up our operations in the near future. Because we have both Windows and Linux servers, we also wanted something that could run on both platforms. Unfortunately it turned out that MongoDB doesn't perform well on Windows, but you can still use Windows servers as replication targets which is great.
Given our C# code base, the fact that MongoDB has a couple of somewhat mature C# drivers was definitely a factor.
How long did it take to train the team to MongoDB?
Did you have a rollback plan if MongoDB is not good enough?
What were your requirements that made MongoDB better than SQL Server and the other DBs?