32bit
wget http://packages.sw.be/rpmforge-release/rpmforge-release-0.5.2-2.el5.rf.i386.rpm
rpm -Uhv rpmforge-release*.rf.i386.rpm
64bit
wget http://packages.sw.be/rpmforge-release/rpmforge-release-0.5.2-2.el5.rf.x86_64.rpm
rpm -Uhv rpmforge-release*.rf.x86_64.rpm
Now that the rpmforge repository is installed, you can install the htop package easily via yum.
yum install htop