$400-500 puts you in the range of the R9 290 or GTX 780, but if you're only using a single 60 Hz 1080p monitor, those are somewhat overkill. I'd go with an R9 280X, personally.
That is very debateable though since the r9 290 or GTX 780 would have more longevity. Games are coming out more and more demanding all the time. I agree that a 280x is fine for now at 1080p, but in a year it very well may not be for some games. Just look at watch dogs for an example, or crysis 3. Those games would have to be ran without AA and maybe not even ultra allready.