Motherboard replacements that fit that case are also slow. You are limited by the maximum of 1 GB of pc2700 memory in two 512 MB modules... and it is the memory that makes the biggest difference... Not the CPU speed.
You cannot find a motherboard that will be faster and still fit all the ports and plugs on the case... There are some mighty decent laptops that will be much faster and run at least 2 GB of memory that would be a better choice... Dell, some Acer, Lenovo, Asus... as well as newer Compaqs or HP's.
Prices will be low for the rest of the summer after high school and college graduations are over... and some sellers will give you a significant discount to reduce inventory. The prices will go back up, but with new models that are faster and run 4 GB of memory or more.
Treat yourself to a Windows 7 machine with the potential to run 4 or 8 G B of memory... a quality build will last you anlother seven years.