First off this article
covers performance of 7870, 660Ti, 7950, 670 Amp while this article
covers 7950, 670 Amp, 680, and 7970GHz so you can get an idea of where each card falls.
Talking strictly gaming, you're really looking at two different performance categories with the GTX 670 and 7970 in one category and the 7950 in the other. A vanilla GTX 670 and the 7970 (non-GHz) are roughly at the same performance level
at stock clocks with the 670 being slighty ahead. So that narrows you down to the 7950 or a 670. If you look closely, the 7950 only gives up about 3-5fps to a 670 in the games tested depending on the video quality settings you choose. (make sure you look closely at the video settings they're using.) The 7950 is equal to a 660Ti which is slightly cheaper on average at the moment here in the states. Both the 7950 and the 660Ti can do 1080p at Ultra settings while churning out ~50fps in todays most demanding games.
PS. In non-gaming situations, the 7950 has better GPU compute capabilites
...if that's something you use.
PPS. According to this
you can do a BIOS flash of a 7950a to "Boost BIOS."
Main: i5-3570K, ASRock Z77 Pro4-M, Asus GTX660 TOP, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Hand-Built Wood Case