Overclocking a 640m

Overclocking your graphics card is done through the OS. You should have a graphics utility that came with the card. It may have any number of names depending upon card manufacturer, 'Afterburner', 'SmartDoctor', 'OCGuru', etc.
If your motherboard does not have on-board graphics, do not get too aggressive in your advancements. As soon as graphics issues start appearing, it's time to back off
Hope it helps some

OK, this is obviously a laptop. The biggest problem is that laptops are specifically built for the heat generated by the specific hardware. The chances are good that if you OC the graphics card it will heat the laptop up too much and cause damage.

IF this laptop is a DV series HP I would say DO NOT OC it AT ALL, they have known GPU cooling issues and the PCI-e bus tends to melt at stock on some models, so be wary.

But yes... If you wish to OC, use MSI afterburner and set up the memory and core clock slowly with stress testing between bumps.

You won't see much improvement unfortunately as that card is limited severely by its DDR3 memory.

Try software like MSI Afterburner, although this depends if your system supports it. Make sure that your temps are good and DO NOT EVER raise the voltage. I would say you could get a good 40MHz extra.