What db system would I best use
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I have a very simple database structure (basically only a user table). Users can log in, get an authentication token, and submit some data that is stored in this User table (only a couple of bytes).  
However, many users can do this simultaneously (100k users in one minute, for instance, for a restricted period of time).  

I am wondering what would be a good choice of technology. I'm not afraid to use NoSQL databases or anything, and am trying to end up with something scalable. 
 
I've been thinking about a queueing system, and a task that populates the database.. or would I just need Amazon SimpleDB and not even bother with queueing the messages? Or do I need the RDS solution to get multiple EC2 instances talking to the "same" database? Or not amazon web services at all?
Thanks for any pointers, I'm pretty new at this and want to get some insight in the various trade-offs and what's best for my application.