Whether upgrading the CPU is worth it really depends on how much lower yours is down the line from the highest your laptop supports, how much it costs in relation to a new, more powerful laptop, and whether or not you want to put in the amount of work required.
It really depends on how much knowledge and patience you have as far as anything beyond memory and storage. Replacing a laptop CPU can involve fully disassembling the computer, which takes time and is easy to screw up, as well as plenty of time making sure that the CPU you buy is actually supported by your computer. A higher end CPU than yours could be made for the right socket, but not be supported by the bios, as most computer manufacturers lock down their bios. I know that in my laptop a Core 2 Extreme QX9300 quad core would physically fit, but the HP bios does not support anything beyond Core 2 Duo T9900, as they had a seperate "quad edition" of my model when I bought this, and they use different bios. As far as replacing the motherboard, I think replacement is possible, but not upgrading to a new model like on a desktop. I think alot of laptops have proprietary mobos made to fit in the case just right around components. I know the one in my laptop has protrusions and is even curved at one spot to accommodate for the CPU/GPU cooling fan. Here.. to give you an idea of what can be involved, I will link the guide I followed to get my laptop apart and upgrade the CPU. The only difference between the one in the pictures on this and mine is that mine has a second thermalplate and heatpipe running to the heatsink for the dedicated graphics chip, whose backplate would be where that black section is below the fan on the next to last image. Just flip though the pictures, this is what I went through to upgrade my laptop CPU. http

/www.insidemylaptop.com/disassemble-hp-pavilion-dv6-laptop/
As far as the display? That is hit or miss whether or not there are even better panels available for any given model.