Building a test database from a large/complex source database

I need to duplicate a complex source database to a different architecture (Solaris on Sun Sparc to Linux on Intel).  I only need to copy a portion of the data. We've tried using Transportable Tablespaces, but the datapump export fails and Oracle hasn't been able to determine why or give us a work around.  So, can anyone recommend a good process (a white paper would be great!).

I'm thinking that I can use datapump (which does normal exports just fine) to transfer the schemas.  I can't transfer the data via datapump because of space and time constraints.  Can you recommend a good process for transferring only a portion of the data from the source database?