I recently purchased upgrade parts from newegg. I haven't had an Intel chip since the PII were in early arival. So anyway, when I went to buy a new case for my P4 2.8C setup I notice that some of the cases where noted as "P4 Qualified 250W" Or "P4 Ready".

What does this mean? I can't just buy an ordinary 400W PSU to use with my new P4?