r9 280 bottleneck with AMD A8-6600K Richland 3.9GHz?

They both have similar single-threaded performance. The fx definitely has better multi-threaded performance. If you intend to add an R9 280, the A8 doesn't make sense. The performance delta between them is not that huge on the benchmarks I looked at, however.
They may bottleneck an R9 280 in CPU intensive games, but the fx-6300 is really a pretty good match for the 280. You should be able to game around 60 fps at 1080p with high settings on all but the most intense games.