To overclock a "stock" rom you will need a new kernel I believe. One that supports clock speeds over stock levels. I can only point you in the direction to do more research on your own. Here is one kernel. And another.
Make sure you know what you're flashing before doing so. Ask specific questions in the thread pertaining to the ROM you are running or the Kernel you want to flash and be very specific about what you have, want, and expect to get the best answers.
The rls1.1 you are downloading has an overclock kernel, so if you want to play with it you're ready to go already.