There is a nice blog about tweaking this laptop http://asusk50idx1unleashed.blogspot.com/2010/08/going-from-22ghz-to-24ghz-smart-way.html
This is the CPUs that model can come with Intel® Core™2 Duo Processor P8800/P8700/P7450/T6670/T6570/T5900 : 2.66 GHz - 2.1 GHz,
Intel® Pentium® Dual Core Processor T4400 : 2.2 GHz
Intel® Celeron® Processor T3100 : 1.9 GHz
If you want to try a quad core, you're better off contacting ASUS support directly unless you want to risk killing the laptop.