The cards should work fine together. You can always clock up the slower card if there is an issue. But there shouldn't be any. A good quality 750W PSU is fine for those cards in SLI. I'm not familiar with the Chieftec brand, but from a quick search it appears to be a good quality PSU. A GTX 560 requires about 150W at max TDP. That's about 12.5A requirement on the +12V rail per card. If your PSU has 4 x 6 pin PCIe connectors, you should be OK.