If i were you, i'd just overclock the processor i already have. To upgrade a processor in a laptop (most of the time) isnt hard: A couple screws, remove a plate or two, losen the heatsink, and replace the processor. The down side is, your processor (when it comes to laptops), isnt -THAT- bad. My guess would be, from what youre saying, that your GPU (graphics card) is the low quality piece (and that is, a high percentage of the time, not changable).
Could you post more specs of your computer, maybe? # gb ram, screen size, resolution, GPU, etc?
P.S.: Your CPU is at least 3x faster than a pentium 4. A current 'pentium 4 equivalent' is an atom (1.6gghz) processor, which comes in netbooks.