Hi Small Blaze,
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.http://gpuboss.com/gpus/Radeon-HD-7870-vs-GeForce-GTX-770
This has many reasons why to choose the 770. Only down side of the 770 would be that it's Total Power Draw is 50watts more. So if you have the head space go for it!!
If not, maybe consider another 7870, what PSU do you have right now and is your Motherboard Crossfire compatible? (Assuming it is)