CPU I think you can upgrade the CPU in a Compaq Evo N 1000c... It has a 1.7 GHz.. But how fast can you go?

Upgrading is to replace a part or software with a newer and/or better one. You can just buy and install any component in a PC, laptops are a bit difficult to upgrade, but okay.

I think you are talking about overclocking. Upgrading can be done limitlessly until all the parts are working and compatible.

Overclocking is to configure a hardware component to make it work like a more expensive one. But it will cause it to overheat, and maybe even reboot erratically if not done correctly.

You will certainly get better performance, but you will instantly void the warranty on it and even reduce its life. Even, overclocking requires special and in most cases, very non user-friendly software. You may also need extremely good cooling systems (like good fans and heatsinks). Otherwise it may practically melt the chips.

On a desktop, it is easy as you can buy better fans and there is more space for water cooling systems.

Choose what you want to do.