I don't think that you'll see much of a performance gain by going with another CPU, i.e. even to say, a T9900. Plus, it can be tedious to replace a laptop CPU. What I mean by that is even if you were to get another CPU with the same socket, the TDP (power requirements) might be different and the mobo may not have enough juice to run it. Furthermore, with a better chip comes more heat that the current heat sink may not be able to handle. If I were you, I'd save up for a new laptop, as you'll be saving yourself quite a bit of trouble.
If you really need cpu power, then you are better off with a desktop.
But if your laptop is sluggish, I have two suggestions:
1) Try to remove all the bloatware that has been preloaded which you don't need. Consider a clean install of the OS and just add the components you want.
2) If your hard drive capacity needs are modest, consider changing the hard drive to a SSD. I have done this and the results are amazing. A clean install of Windows-7 will take about 15gb. A 40gb Intel X25-V is really all I need.