Welcome to our forums TheRealFnao. You definitely want to use the cables labeled PCIe to plug into your GTX 760. Be sure to plug in both the 8-pin and 6-pin connections on the card otherwise you will run into a series of problems attempting to power up your system. 
Be aware that in most cases the PCIe cables come in 6+2-pin and 6-pin on a single cable so to fulfill all the 8 pins on the card you will need to combine the 6+2 to make a total of 8 pins just to be clear on how this connection works and it can only go in one way on the card since the connection is keyed to avoid plugging it in the wrong way. 

This ^^^
 
Welcome to the forums..I believe the CX600 has two 6+2-pin PCI-E connectors which you can use as either a 6-pin or an 8-pin, as needed..Plug one of the PSU's 6+2-pin PCI-E connectors into the GTX 760's 6-pin connector, leaving the two extra pins hang..Plug the PSU's other 6+2-pin connector with the two extra pins into the 760's 8-pin connector..Use only the connectors labeled as PCI-E.

Just to be clear, you did also plug the 8-pin connector labeled CPU into the motherboard as well? I only ask because if it was plugged into the MB, it wouldn't have been an option for you to plug into your GPU...

- Yes you can enter BIOS without a HDD with the GTX 760 installed (no OS or drivers needed)
- You would need to buy a laptop to desktop hard drive adapter if you wanna go that route, you can search  online for one.
- The 8-pin (should be a 4+4-pin) connector from your PSU that  is labeled CPU on the side goes to the 8-pin mobo connector that's labeled ATX_12V.
- Your mobo manual will tell you where the connectors go..Connectors are   keyed so that they only fit properly into what they're supposed to fit   into..Don't force them.
- Your PSU manual will explain what connectors go to what.