Go to
http://ts.fujitsu.com/support/
Enter the serial number.
Remember that you can only use Intel P4 CPU, and probably not just any P4 build. The new Core CPU are NOT compatible (no i3, i5 or i7).
If you want to keep your current computer, investing in more RAM or adding a new, and larger, hard drive will probably be easier and more effecient performance wise.
If you want to upgrade because you don't have enough CPU horse power, then you are probably beter by geting a new computer.
Electro, April 2011