Even with the standard GTX570, people have no problem overclock a GTX470 to average 820MHz+...you'd have to be
extremely unlucky to land a card with a core so poor that can't overclock to 800MHz (at least I have not yet seen anyone that couldn't overclock their GTX470 to over that). Even at just 700MHz (stock clock is 607MHz), it would be around 5870 speed.
For overclocking graphic card, the most commonly used program is the MSI Afterburner. Unlike overclocking CPU which require playing around in bios and changing lots of settings, overclocking graphic card is software based and it's so simple, all you have to do just grab the bar for the core clock, shader clock, memory clock (or enter the values directly) and press save, and then choose 1 of the 5 avaiable profile slot you wish to use that
that's it! But for high overclock, you gonna need to increase voltage to keep the overclock stable, which would require you to unlock voltage settings in MSI Afterburner before you can increase the voltage:
http://event.msi.com/vga/afterburner/gui.htm
As for what voltage/setting to use or start the initial overclock, you can make a thread on this forum and ask other people that own GTX470 for advices. MSI Afterburner also allows you to set your own fanspeed profile settings (for the sake of noise and temp control), which basically you just adjust the line on the "line graph" of what fanspeed at what temp to use i.e. 40% at 30-40C, 40% at 50C, 50% at 60C, 60% at 70C, 70% at 80C etc. For the MSI Twin Frozr II cooler, I believe it should be unaudiable up to 60% speed, and then above that, you would probably start to hear it, but probably the the sound of wind/air moving in a non-intrusive way...it would probably only start to become loud when the fanspeed get to 70% and above...but I doubt the temp would get to that high, so the fanspeed should never reach that level of speed anyway.
Anyway, the 6950 SHOULD be out on 15th Dec, so there's no harm to wait till then before deciding which card to get. But just a reminded that 15th Dec Wed is also the day which OcUK would change their "this week's offer" to other items, so the discount for the MSI GTX470 Twin Frozr II would be gone by that day.