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.
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.