the dual core patch is meant for some few systems where cpu´s with more than one core are running software slower than usual.
It´s not meant to help to fully load both cores. That can only be accomplished by the software.
Thats why mulicore systems do not make much sense for home users yet (in terms of playing e.g.).
You have to buy software which is optimized for multi cpu systems to get both cores to work under load. Some mulimedia software is already optimized, most games are not. Winrar e. g. shall use both cores since the latest version.
edt.: If you dont see 2 cpu´s at the taskmanager you might have not installed the chipset drivers. Upgrading to a new multicore cpu needs either to uninstall the cpu-/chipset drivers before installing it or could also be fixed by a repair install of XP if just uninstalling of the necessary drivers does not fix the problem. If this does not work as well (which is unlikely) you might need to reinstall XP.
I once made a repair install and it worked properly showing 2 cpu´s and without causing crashes, but the installation of the new chipset- and cpudrivers is much faster (uninstall the old drivers first!).
But you can run more applications and assign different cores to each application which is very useful as well since some patches or new software arrives.