I'm wondering the exact same thing ! In the UK right now 670's are £209.99 , SLI is £420 (obviously) 780 is £499 ! Either way I'll need a new PSU too my 500 Watt wont cut it . Kicker is I game at 1440p unlike the OP .
I'm leaning towards 670's as SLI and a new PSU will equate to one 780 ! Thoughts.......?