If you are getting a cheapy case, I wouldn't trust the PSU that comes with it, if one does. If you are getting a case from a reputable manufacturer (eg Antec), then the PSU should be good. If getting a no brand case, I get rid fo the PSU that case with it and buy a quality PSU.
Like CookieRevised said, 300 W will be enough, unless you want to run some crazy lighting off it to show off your SLI PCI-E video cards
, but if a yumcha PSU
says that it can output 300 W, it might not be able to.
Summary
If buying a case from a trusted manufacturer, the PSU will probably be 300 W or more, and you'll be set. If buying a dodgy brand case, the rating that is given for the PSU is probably false, so don't pay attention to what rating is given, or whether there is a PSU at all, and buy a quality PSU to use instead.
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