Do cases ome with their own power supply?

[MasterChef]

just a stupid question...

[Rick]

Depends on who makes them and where you buy them from.

NO Reason to buy one with a 350 watt PS if you need a 450watt

Shop around.
Find the case you like and if available with the correct PS grab it.

It has been my experience that Most don't come the PS I need or want

A few dealers will allow you to order with a better PS for less than if purchased alone

[Whyzman]

Here's an excellent read that will help determing PSU needs:

http://www.firingsquad.com/guides/power_supply/

As Rick pointed out, "Most don't come with the PS I need or want." However, it has been my experience with the cases I purchased that the PS was almost tossed in for nothing...I get the combo and swap in the PSU that I really want...keeping the "gimme" PSU for troubleshooting. ;)