There is a server hook script available at: http://svn.collab.net/repos/svn/trunk/contrib/hook-scripts/ that will prevent checkins which result in case conflicts.
If you already have two files with a name that differs only in case, you have to decide which one of them you want to keep and delete (or rename) the other one from the repository.
There are (at least) two possible solutions to rename a file without losing its log history. It is important to rename it within subversion. Just renaming in the explorer will corrupt your working copy!!!
Solution A) (recommended)
Solution B)