Don't overclock the CPU, unless you put the CPU fan to "always on" and if the CPU is still within safe temps during 100% load 1 hour, during summer.
Depends on how slow your machine is, a fresh install will help, but if it's got 128MB it'll still be quite slow. Really 1GB RAM for a laptop is desireable now. Since laptop hard drives are quite slow you want as much RAM as possible.