Is the R9 270 still good enough or is it time to upgrade to another card? Most games I see it seems to run everything on Ultra but now there are a few like Witcher 3 which seem to only let it run on medium.
Main question is how badly you need new GPU, if you can wait few months wait for GPUs on 16/14nm node 2x more performance per watt
If you want go earlier try look for local deals and second hand offer ebay ect. you can get cheap R9 280/7970 R9 285/ R9 290
With that graphic card you will run witcher 3 on medium or high if running at 1366x768 , you should upgrade since you can get a better graphic card at cheap price , what is youre cpu?
Main question is how badly you need new GPU, if you can wait few months wait for GPUs on 16/14nm node 2x more performance per watt
If you want go earlier try look for local deals and second hand offer ebay ect. you can get cheap R9 280/7970 R9 285/ R9 290
You will no need new mobo for new GPU
PCIE slots are standard and they will still be in use, but for how long who knows but for sure for next 2-3 years