If you really need very large scale SSAS size/performance I would definitely stick with the newest edition of SQL Server you can move to. They have improved scalability and manageability significantly in the last few editions, and I think this continues in SQL 2012. Do note however, that much of the BI underpinnings is changing significantly in the latest version of the SQL Server ecosystem.

Best of luck with your project - sounds like fun! Drop me a line if you need some help with the relational engine side of things, or a second (NON-SSAS) pair of eyes. I couldn't SSAS my way out of a paper bag with both ends open, but I can make the relational engine spin like a top! 😎