I see you haven't been following whats being going on lately
I haven't being following with an eagle eye but last I saw the 7970 should be releasing at the end of the month and should be available for around £450 - £500.
Now that I look at it two decent 560 ti's should set you back around £350 - £400 total. Bit more of a difference than I thought.
Until the 7970 comes nvidia's GTX 580 is the current single GPU king. A 1.5GB model will set you back around £400 and a 3gb model will set you back around £450. It is suspected that the price of the 580 will fall shortly after the 7970 is released as the 7970 beats it hands down. The price drop is only speculation though. AMD's lower level cards should be releasing soon after and depending on there performance nvidia may have to start reconsidering the price of the rest of there cards.
Nvidia will not be releasing there next generation of cards until late 2012 - early 2013 (No concrete evidence of release dates though)