Just get a new case. That model is a compact type system. They removed the darn serial / parallel ports to make it smaller.
While it's nice to have small computers, you sacrafice size for performance. If you don't have a lot of cash to dump in a case, check out www.colorcases.com
or any other online dealer. But I suggest upgrading.
Another reason is this, you'll need a new power supply for a P4 or Athlon. If this IS a micro AT/ATX case, then you won't be able to fit a regular 350w power supply into it.