The PCIe revision for the 760 is 3.0. The PCIe revision for your processor is 2.0. That may seem like an incompatibility, but no GPU currently on the market can saturate a PCIe 2.0 slot, and PCIe is backwards/forward compatible, so NBD.
I agree that it would not be worth your time or money to upgrade with less than a 760. However, if you have two x16 PCIe slots on your motherboard, you might also consider buying a matching 560 Ti (same make/model/edition). I think they run as low as $130 now. 700W seems like it would support a second Fermi GPU at stock speeds.
If you haven't overclocked your 560 Ti, that card is remarkably overclockable. My 560 Ti AMP! performance falls right in the gap between a 560 Ti and a 570, in some games edging very close to 570 stock speeds. I can still get framerates in the 40s on medium-high settings on DX11 games on 2560x1440, so I, too, am waiting until the 800 series to upgrade.
Another thing to consider is that the performance increase between architectures is greater than the performance increase between series of the same architecture. 500 is Fermi, 600/700 are Kepler architecure, and 800 is Maxwell. So 500 to 600 was a big performance increase, 600 to 700 was a smaller performance increase, and so 700 to 800 should be a big performance increase.
It sounds like you've gotten a lot of mileage out of your card, and if I were in your shoes (I am in your shoes, really) I would put aside $250-$300 for a new card in case your current one fails while waiting on the 860.