If you're willing to spend that much, your best bet may be to simply buy a new computer. For $300-$400 after rebates you can have a new system with a 64-bit AMD 3400 or better processor, 160GB hard drive, and occasionally even 1GB of RAM. You could give the new one to her and then clean out the old one and use it as a secondary computer.
Other thoughts include:
1.) What tasks is she performing? That computer may just need a tuneup depending on the task at hand.
2.) Are you sure that computer can even be upgraded to a 3.0GHz Pentium 4? Name the make and model to be sure.
My daughter has a desktop system that's several years old and is now relatively slow. It's got a 1.8 GHz Pentium 4 CPU and 512 MB memory. I could replace the CPU with a 3 GHz for about $100 but would that alone be enough? More memory may or may not help. I'm willing to spend several hundred dollars, but how can I get the most bang for my buck? Thanks.