Yeah, the HP Pavilion line is a mainstream setup. More than likely not possible to change out the GPU as it's built into the motherboard. And I highly doubt they designed it to accept a discrete plug-in GPU. You have to specifically buy a specialty gaming laptop to be able to insert your own GPU.
Couple manufacturers actually sell laptops without a GPU in them so you can do just that.
But those GPUs are very spendy.
Here's an example of a worthy gaming laptop:
MSI GX640 (Core i5 450M + ATI 5850 + 4GB RAM + 1680x1050 res) $1,082!http://www.xoticpc.com/msi-gx640260us-p-2766.html