Fair do's, release dates fluctuate, the last official release from ATI says its all on track for 2011 release - but worth the wait regardless (im still running 2x 4870s in crossfire btw)
7900 series offers 100% more performance than a 6970, and also a little bit more than two 6970's in crossfire - that's a serious reason to wait if you ask me.
If you dont want the new tech, that will also be the right time to pick up a dirt cheap 6970 / 6990s - not now.
At this point in time, you are still paying primo money for a card that has a few months left.
7970 spec -
Stream Processors 3048
Texture Units 128
ROPs 64
Core Clock 1020MHz
Memory Clock 1.7GHz (6.8GHz effective) GDDR5
Memory Bus Width 256-bit
Frame Buffer 2GB
FP64 1/4
Transistor Count 3.2B
Manufacturing Process TSMC 28nm
1x6,1x8 pin power, TDP 250W
Incidently OP, the amount of onboard GDDR has much more impact on resolution / screen size / multiple displays, rather than the clock speed of the GPU.