Forget any "gpu horsepower" concerns, a r9 390 will work great for a 1440p monitor, I know first hand as i ran my monitor with a gtx 970 with no problems at maxed or nearly maxed settings.
1440p is a great choice for monitors right now as 4k is still in its early phase and has some teething pains.
I have and can highly recommend this Acer monitor Here.
Its a 25 inch monitor which is slightly smaller than normal for 1440p but the IPS display is beautiful, no dead pixels, mine overclocked to 85hz easy, the lack of bezel on 3 sides looks sexy and the price is really appealing.