A six year old computer is the device to start overclocking with. What do you have to lose?
Overclockable computers usually have aftermarket motherboards with some special features that allow the user to overclock the processor and memory in the BIOS. Your motherboard should have the ability to change the memory frequency, timings and voltage. The board should also be able to change the processor multiplier, frequency and voltage, and it should be able to raise the chipset voltage. You'll also need memory that is capable of running faster than its rated speed or a motherboard that has memory dividers.
A simple overclock would require you to raise the front side bus. As you raise the bus the memory and processor frequency would also raise. The memory would raise 1MHz for every one 1MHz you raise the FSB as long as there wasn't a memory divider put into place. If the processor has a multiplier of 7.5 (like yours probably does) then the processor speed would raise 7.5MHz for every 1MHz the FSB was raised.
Does your BIOS allow overclocking?