Double your memory and your SSD:
1866/2133 is pretty cheap now iirc, or even 1600 is 'fine' if you're budget doesn't stretch.
GTA 5 is about ~50GB install which will eat your 120GB drive. 240GBs are pretty affordable these days.
Look at the R9 285 or GTX 960. Both are faster than the R9 280 and shouldn't be a big price diff. Ignore the fanboy ranting about '128bit' on the 960 - it's more efficient and the in game results performs just as well.
To offset the cost:
Drop the PSU to ~600W, that's plenty for 1GPU and two drives. I would go as far to say even a quality 500W is fine on that build, but it doesn't leave much room for upgrading if you want to go multi-GPU in future, perhaps. The Seidon 240M is lovely and quiet - I use one - although if you're not into overvolting you can get away with 120M on that chip, even with some OC.
