It depends wildly.
For ATI you got the advantage of a company with lower prices because they are trying to catch back up.
For Nvidia you know you are getting the best of the best but not always for the cheap.
So unless you are ready to spend 300 USD I suggest going ATI.
The ATI card you listed looks REALLY modern (HDMI on card) Do be aware tho that it's HDMI audio may be only optical rerouted and not the dedicated copy protected path that gives you full features of a dedicated Blu Ray player (This only applies if you intend to watch blu ray tho)
BTW to get a sense of what the numbers mean I suggest looking at the wiki articles.
http://en.wikipedia.org/wiki/ATI_Radeon
http://en.wikipedia.org/wiki/GeForce
For power supply the thing to look for is watts.
http://en.wikipedia.org/wiki/Watt