Corsair RM550 should have:
(a.) 6+2-pin PCIe (you should have 2 nos. according to the label)
(b.) Serial ATA plug
(c.) 4-pin (FDD)
(d.) +12V 4+4-pin (EPS12V)
(e.) 4-pin (HDD/ODD)
and so on...
If only you have a single cable like item (a), that's fine as long as it has short loop like the picture I shared earlier...
Then you can still connect it to your graphic card (like GTX760) that requires 6-pin and 8-pin connector.. its just a short loop or like a jumper cable from the same voltage in same rail of psu.
In this picture below, though the connector doesn't looks like your GTX760, still theoretically it the same
Theoretically, the short loop or jumper will have the same amount of amperage through this connection. 6+2 wire connecting it to a 6-pin socket won't overload anything as long as your psu can handle the required power, in your case according to the label it has 2 PCIe plugs.. I am not sure why you got one. Anyway, let say:
6-pin plug has max. = 75W,
Pcie Slot (motherboard) = 75w
8-pin (or 2+6-pin) = 150w
In your case, GTX760 requires 6-pin (75w) + 8-pin (150w) & Slot (75w). Total Max 300w.. But actually GTX760 power consumption ranging to 168w under load.
http://www.guru3d.com/articles_pages/geforce_gtx_760_evga_superclocked_acx_review,5.html
Different gfx card designers have their own boards design, factory over-clocked GPUs and cooling system.
The power your graphics card usage varies from the reference power requirements.