Thesios :
Firstly, can i still use the S340 Mid Tower case? I prefer it and its cheaper.
Definitely! The Phantom 410 has better airflow which is why I swapped them.
Thesios :
Secondly, is the R9 280 going to be sufficient for the type of gaming I'm doing? I want a good graphics card, but i'm really wary of overdoing it.
I don't know what type of gaming you're doing. For 1080p, it'll do very well. The R9 280 is the most reasonably priced GPU in Australia, in terms of AUD to USD conversion. The NVIDIA cards in Australia are really expensive.
Here's some benchmarks you can look at: http://www.bit-tech.net/hardware/graphics/2014/05/19/amd-radeon-r9-280-review-feat-xfx/5