Ultimately it depends on what games you want to play, how high you want to set the graphics quality and what you consider minimum acceptable frames rates are.
For example, an OC'ed Radeon HD 7790 is sufficient for Skyrim since the below benchmarks shows an average FPS of 70. That could mean that perhaps the lowest FPS you will get might be 50. While in Borderlands 2 the OC'ed Radeon HD 7790 would be considered overkill since the average FPS is 111; try see that many frames on a 60Hz monitor. However, Metro 2033 is a completely different story. The OC'ed Radeon HD 7790 will only give you 33 FPS on average. If you want at least an average of 50 FPS in that game, then you will need something faster than a Radeon HD 7870.
Note: The test rig uses an OC'ed Intel Core i7-3770K @ 4.6 GHz. 1366x768 resolution is not benchmarked, but 1280x800 resolution is and the difference in the number of pixels is about 2.5%.
Source:
http://www.techpowerup.com/reviews/XFX/HD_7790_Black_Edition_OC/1.html