I would recommend, if you don't want to get a new PSU and OC your CPU, getting a GeForce GTX 770. It's a very good graphics card and is much better than the 7870. You're very right in saying the 7870 is perfectly fine but if you do want to upgrade and don't have a problem changing to nVidia that would be my recommendation.
Since the prices of 7870's is still pretty high I would tend to agree with Liberatas and just get an ASUS GTX 770 DCUII http://www.amazon.com/gp/product/B00D3F7CFK. You won't need to upgrade your power supply and you will be able to sell your 7870 for around $200 which cuts your cost of the 770 by over half.
The GTX 770 is also a great performer and should be able to nearly max any newer game at 1080p resolutions.