Yes, a good 550W would be enough for current specs and for a minor GPU upgrade ( maybe as much as a 6970. ) And if you're planning on staying with current specs for a while, there's no need to go higher.
If you're planning on a major GPU upgrade in the near future, you'll want a little more power, just to be safe. Again, you don't need to go bonkers on the PSU. I can CrossFire 6870s ( ~300W draw, ) with my 2600K on my 700W PSU and still be safe ( even with a little kOCing. )
Most people tend to go overboard on PSUs when upgrading ( though a lot also get too little, but that's usually because they get a shoddy PSU. ) A quality PSU ( Antec, Corsair, the usual suspects, ) in the 700W range is more than adequate for any system using one high-end GPU or dual mid-range cards ( the
December SBM CrossFired 6950s on a Corsair 650W. ) And with the 7000 series lowering power requirements, I don't see why you wouldn't be able to CrossFire 7950s on a 700W PSU as well.