very large RAC database copy to another RAC

I need some thoughts on this. We have a very large NOArchivelog database 11.1.0.7 RAC 20 node RHEL4, 300+T spread on about 50 tablespaces, we want copy this to another RAC cluster RHEL5 30 nodes in a different data center connected with pretty good network infrastructure between them. They both use ASM of course.

We are probably allowed to be down on the source for about 48 Hours max.

So our aim is to parallelize(increase throughput by utilizing all the nodes of both RAC systems ) the copy from one ASM to another ASM directly. I want to know what techniques are available to do this. Few techniques that came up in the discussion are RMAN duplicate database, dbms_file_transfer via dblink to copy datafiles using transportable tablespaces technique etc.

I want to know if any of you out there did dbms_file_transfer between databases and if you did, can you please share your experiences.

Any thoughts/pointers are greatly appreciated.

thanks
Deen

--
http://www.freelists.org/webpage/oracle-l