Either 670 or 7950 imo, neither the 7970 nor the 680 are worth the price difference between their two slightly cut brothers.
 
Titan is a joke for perf/$, it's only for people who want to absolute best performance possible or need the DP for CUDA.
 
7950 can OC to around 670 performance, it will use more power though for similar performance. 7950 has 3GB frame buffer, vs 2GB for 670 not a major factor at 1080p, but with PS4 having 8GB share it may become one.
 
670 should hold resale better than the 7950, price difference between two is noticeable. However with the 7950 to get decent thermals and acoustics and unlocked voltage you're going to want to spend around $300, with the 670 you can either go reference or with a $360 variant... OC doesn't do much for the 670, card is really best used stock imo.
 
You get metro last light with the 670, while you get Crysis 3/Bioshock Inf/Far Cry 3 Blood Dragon with the 7950... Games shouldn't be a huge factor anymore, most of them except metro are cheap enough to find, you can either buy the codes from AMDs promotion from other people or just use cdkey websites around the world to get really good deals (hooray PC gaming).