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Oracle RAC is the Rolls Royce of databases allowing extra hardware nodes to be added relatively easily and hardware failover.
However, your commodity hardware costs will be dwarfed by the licence costs.
Why dod you feel you need horizontal scaling. A multi CPU core server with 40GB RAM and SAN storage can support very sizeable DB installation.
Can you provide any sizing and expected activity information to allow better understanding of your problem?
If you do go down the RAC route it's worth remembering that it doesnt scale horizontally forever. Even the sales guys admit 90% of rac customers are 4 nodes or less. Once you go more than that you get diminishing returns. So rac may work for you, but it's not guaranteed to be the answer.
MySQL: http://www.mysql.com/why-mysql/scaleout.html
Limitations are that it works best with read-mostly workloads. You typically have one 'master' that receives all the writes, and many 'slaves' that replicate the writes. Then you distribute the reads over all the databases.
MySQL replication is asynchronous, so you will probably have to deal with time lag problems (you write to the master, and then read from a slave before the write has been replicated).