Oracle RAC is not horizontally scalable at all, because all Oracle instances share the same data storage. Yes, with SAN stuff u can get a large size DB, but it's just not scalable at all. In other words, Oracle RAC is still a scale-up approach. So for scaling-out or horizontally scaling, you have to partition your data by function that means put different groups of tables in different databases; or partition your data per table that means partition one table into multiple subtables with the same schema but store in different databases. In this way, you get a scaling-out solution. There are many resources on that. Sharding has been a buzz word for a while in web 2.0 website architecture blog sphere.
Because Sharding is not directly supported by database itself, you have to build your own solution. But as I said, there are many lessons already. For oracle, partitioning table is possible. For mysql, check this question
