Technically speaking there should be no problem with the choices you are making. However, that all depends on the level of experience the developers have with Seam/JSF/Richfaces well...all your technologies for that matter. I consider myself more advanced in my knowledge of Seam and JSF then I do with JPA/Spring (although I do use them all at the moment) So I'll focus more on Seam and JSF. Beginner examples are full of awful code that will severely impact the performance of your application.
Understanding articles such as this one that teaches you how to speed up your JSF application will be crucial. Understanding how to effectively use @BypassInterceptors and other Seam features will also be very important. The BypassInterceptors when used right can make a significant improvement to your application's speed.
So, I think the biggest question you need to be able to answer is how much experience does the development team have with the technologies.
400 concurrent users is going to provide you with a lot of challenges all the way down to database design. So, make sure you have the right people for the job. :)
Go for Seam 3 and AS 7.
Its easy to add a cluster and handle large amount of users. :)