Fobotcso,
In general, it can't be done. Manufacturers do not create "motherboard upgrades".
The closest scenario that can work: a particular notebook brand release a series of products that all use the same chassis, same case.. same LCD... same internal physical connections. Say the only difference between the various models was CPU power.. apart from HDD/DVD/CD specs. In that case, you could upgrade the motherboard to a faster model.. but would be paying through the nose..
Note: in most notebooks, CPU and motherboard are one unit.
Unfortunately this rarely happens. Most "faster/better" models come with different video cards, different LCD's.. so when attempting to install the upgraded motherboard you'll find that the video card connections don't match, or the LCD connector is a different size etc. etc.
Hope that makes sense.
Lachlan