Replaced Pentium II 300 with Pentium II 450, still running at 300 mghrtz

Hi,
I have a computer purchased back in '98 running a Pentium II 300 mghrtz chip. I recently purchased a Pentium II 450 and installed it, however the computer still is only running at 300. I know that jumpers and/or bus speeds need to be changed, but I don't know how. I looked all through the BIOS and can't find anything that refers to bus speed. I have changed the jumpers around but the system runs at 300 no matter what the jumpers are set at. The motherboard is a M6TLC, and I have the manual, but cannot seem to find out how to change the bus speed. There are jumper setting diagrams, but none that refer to a 450 mghrtz processor, and changing them does nothing anyway.
I wanted to up the speed on this old machine, put in a new bigger hard drive, put WINxp (it now runs 98se) on it and use it for backups. I'm beginning to think that this processor upgrade can't be done, but any advice would surely be appreciated.
Thanks,
TennisWitch