sounds like you have mysql version 4.1.20-1 installed, so try removing it first with
rpm -e mysql-4.1.20-1.RHEL4.1.i386
then try insalling again with
rpm -ivh MySQL-server-standard-5.0.27-0.rhel3.i386.rpm
but before you try that second command, what distro are you using? Most have some tool to make installing things easier. Fedora has yum, Debian has apt, etc.