Latitude C840 cpu upgrade

I have a C840 running XP which does what I need it to do. I thought I would try a cpu upgrade, I've never done a PC before. I got an Intel SL6DW-U30 cheaply and fitted it.

The BIOS still shows 1.89GHz and not the 2.5 I was expecting. I tried fiddling with the power settings from the desktop but that didn't help. There doesn't seem to be anything I can change in the BIOS unless I have missed something.

Aplogies if this is a really schoolboy error but I would like to get the new cpu working at it's proper speed.

Thanks

              Intel(R) Pentium(R) 4 CPU 2.53GHz
x86 Family 15 Model 2 Stepping 7, GenuineIntel