Balancing Data Between Files

We are storing all our SQL 2000 databases on SAN LUNs, and one of our databases currently uses a single 40GB file which is approaching capacity. If we add further files using different LUNs, the data will start being added to these new files. My questions are these: if we were to add a number of new LUNs to this database, is there a way to redistribute the existing data so it is balanced across all files in order to gain the most benefit from having multiple files, rather than just dispersing the additional fragments across the new files?
Will the optimise feature of the maintenance plan do this automatically during the index rebuilds?
Is it better to add more files to the PRIMARY filegroup, or add a number of filegroups with single files in each? We aren't looking to use filegroups for fiddling with our backups by the way.
Many thanks for any recommendations offered.