The FX-6300 is a bottleneck at times. Lots of people seem to think that's not the case. Overclocking will help though.
It does however VARY between games how much the bottleneck is. I still recommend you get the R9-280X but I thought you should be informed in case you wonder why you don't get the same performance in many games.
I'm not sure why there's such confusion, but the CPU is deemed a bottleneck in a computer if you replace it with a BETTER CPU and performance is better, as in a different frame rate.
SKYRIM - The FX-6300 will be a bottleneck.
http://www.tomshardware.com/reviews/fx-8350-vishera-review,3328-14.html
Above, just an example but if the FX-8350 (a better CPU than FX-6300) wasn't a bottleneck then the i5-3570K wouldn't beat it. (The graphics card used was a GTX680 which is almost exactly the performance of an R9-280X)
BF4 Singleplayer - The bottleneck should be very minor.
http://www.techspot.com/review/734-battlefield-4-benchmarks/page6.html
Above, only the bottom three CPU's pose a significant bottleneck.
BF4 Multiplayer - Probably a slight bottleneck for average, and a few major drops for the LOW scores but I have no data.
WHAT CARD?
Possibly THIS one:
http://pcpartpicker.com/part/msi-video-card-r9280xgaming3g
Benchmark:
http://www.techpowerup.com/reviews/MSI/R9_280X_Gaming/26.html
NVidia alternative: Asus GTX770
AMD:
- Mantle
- this card has 3GB (may or may not use it in the future)
NVidia:
- PhysX
- G-Sync (for G-Sync monitors)
- Shadowplay
Many people like Shadowplay which allows Game or Desktop recording/streaming. Works very well.
Cheers.