if the GTX 560 supports tri-SLI or Quad-SLI
then might be cheaper to pick up another GTX 560 to improve the performance at lower cost, rather than spend minimum $1000 on a GTX 690 or HD7990
you actually pay more if you got double GTX 680 or double HD7970
it's at $169 on newegg
so $338 if you get two
now the sad part, if your motherboard doesn't have a 4 way SLI... then you'll have to change the motherboard
since you have a i7 920, that means you are probably on the LGA1366 socket
(i still have one of those laying around)
it's quite a challenge to find a kick arse motherboard for such an old architecture...
(the CPU itself is more than capable for any games out there right now and even upcoming unreleased games, so there's nothing wrong with that)
check ebay and see if you can find any of these:
EVGA 170-BL-E762-A1 LGA 1366 Intel X58 4-WAY SLI
EVGA E758-A1 3-Way SLI LGA 1366
ASUS P6T7 WS 3-Way SLI LGA 1366
there are probably other models that i forgot, but that should get you a good start
-- Edit ---
a quick search on ebay reveals the ASUS P6T7 WS for aprox $200
so going tri-SLI will run you up for under $400