Laptop overheating while gaming...Which cooling pad to buy???

Laptops, especially the HP Envy series of which I own one as well, have heat issues. Period. All of them. Overclocking destroys the delicate balance that is configured in to the design. Gaming makes it worse. Laptops are only designed to provide cooling that is sufficient for the design and altering the design tends to create issues, as you've seen. I'd recommend NOT overclocking ANYTHING, and hope that you haven't already damaged any hardware.

A cooling pad will do NOTHING for a laptop that is overclocked or used for gaming. Well, it might help slightly for gaming but it's not a desktop and there are no cooling products that are going to give you the ability to overclock safely or in most cases, game at a high level for long periods.

Laptop hardware is not as hardy as desktop hardware, and less forgiving when it comes to heat tolerance. Overheating your system once or twice on a desktop is probably not going to destroy it. Doing so on a laptop, might, or might not. The Envy series laptops also are prone to fan failures where the fan no longer achieves high speed operation, not going past a medium speed and not spinning up to full speed when you first power up, then slowing down to normal like they did when new and when that happens you will need to replace the internal fan which is not for the faint of heart, but easily done if you are familiar with doing these types of operations.

Probably you just need to remove your overclock and reduce your expectations. If you want a high end gaming machine, get a desktop. Otherwise, the sustained demands of gaming WILL eventually degrade your unit to the point where it overheats even during normal use if it hasn't already. Probably any half decent cooler will help for the simple fact that it gets it a bit higher off the surface it's on and does add some measure of airflow movement around the bottom of the case. Don't expect much though as the areas that tend to overheat, being the CPU and in some cases where there is a discreet card installed, the GPU, are unlikely to be affected by the addition of a cooling pad as it will not directly exchange heat from the internal heatsink.