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A _GOOD_ 400W PSU would be sufficient for most single-GPU cards out there.

The problem is there are many cheap PSUs that only deserve about half their nominal rating so the specific PSU model is necessary if you want to know whether or not it can be trusted.

In theory, 500W is enough for any single-GPU card out there but mileage may vary due to many 500W PSUs not being worth 300W.

as the above comment says a quality 400w PSU could handle everything.

Do mention your GPU & your budget.

When someone who only seems to have a vague idea what they are doing, I simply go with a minimalist worst-case: most powerful single-CPU, single-GPU configuration. That comes out to about 350W. Since PSUs are generally most efficient up to ~70% load, divide that by .7 and you get 500W.

Since the OP has an i3-550, I doubt the HD7970, 680GTX or anything else of the sort including SLI/CFX are on his menu so any decent quality 500W PSU should be vastly sufficient for anything he might do with his current platform.

Well, I never contradicted what you said. And yes, a good quality 500w would be sufficient for a single GPU rig.
But, if he owns a Mid/Low level GPU(5750/6750), then he could run his rig even with a 350w PSU.(Antec VP350W)
IDK if he's looking to buy a new PSU or not, but if he has a new GPU & wants to understand his minimum PSU requirements, then whats wrong in knowing which GPU he's running (or planning to buy) & if he would want to buy a new PSU, then understanding his budget would also be good.
I asked about his GPU & budget mainly because, I've come across many users who were looking for a very cheap PSU.

There's no way to know the correct answer without knowing ALL of the OP's components - period.

You guys are ass-u-ming.

PSU Calculator - http://www.thermaltake.outervision.com/Power

Recommendation - too large is a hell of a lot better than too small. To small and the reduced lifespan, PSU bottleneck, failure rates not to mention noise is a very poor choice. Further, stick with a good PSU e.g. Corsair, SeaSonic, or Enermax.