Gigabyte GTX670 OC =
$400 (OOS right now)
Gigabyte HD7970 =
$499
Now look at HD6970 vs. GTX580. At the time GTX580 cost $500 and HD6970 cost $370. Now look at the performance increase from HD6970/580 to HD7970 @ 1250mhz/GTX670 @ 1330mhz in that VRZone review you linked.
It becomes pretty evident the $100 extra spent today for 5-6% more performance from 7970 will not make games any more playable. However, that $100 will net a HUGE performance increase next time it's put towards a next generation videocard.
Same thing happened with HD4870 vs. 280, HD4890 vs. 285, HD5870 vs. GTX480, and will happen with GTX670 vs. HD7970/680.
No one can tell you if $100 savings is worth 5-6% extra, but based on how last generation of cards fare today, I am confident that HD7970 will be obsolete just as fast as GTX670 will be (and same for 680). In 14-15 months people will have upgraded to GK110 or HD8000 which might be 25-40% faster than today's cards and provide a tangible upgrade.
Rephrasing the question: Is 5-6% extra speed that 680/7970 OC over GTX670 OC worth $100 today?
.