You haven't specified what questions you are trying to answer with your queries, or how your data is structured. Before you choose what solution to use you probably need to think about those two things.
You're correct: the major RDBMS vendors offer clustering solutions; both for parallel processing and high availability. They've had this technology for a while and any enterprise with a lot of data is probably using it. When you buy ($$$) the product they will give you lots of documentation and help you set it up (more $$$) if you can afford it.
RDBMS are good for online transactions (OLTP); answering questions about specific rows (where does Mary live?); answering some summary-type questions (how much did we sell in the first quarter, etc.) Although they can be made to perform detailed summary questions (how much did we sell in the first quarter, broken down by product, salesperson, month, and region?), you're usually starting to tax their limits (any query that needs to visit all of the rows is going to be slow).
For those types of queries most enterprises have a data warehouse that structures the data into multi-dimensional "cubes." (See Cognos, Hyperion, others). That may be appropriate for what you're trying to do.
I don't have any experience with MapReduce but I've read the wikipedia section on Uses and so if what you're trying to do falls into those categories I'd continue with it.