With respect to the mods but the recommended power supplies are way to powerful for your components. It's a common thing to go overboard with power supplies. Obviously they will all do the job, but you waste a lot of money.
Your system without being overclocked will draw no more than 300W from the wall at MAX(!) load. Now we know that powersupplys work most efficient between the 50% and 65% range. To put it into context your MAX load wattage should be MORE than 50% of what the powersupply is capable of. So a good choice would be a quality ~600W power supply which still gives you plenty of headroom for OC.
To back things up for you I did a quick search about your components and found a review from HARDOCP. Note that the CPU is more powerfull than yours and overclocked to 4.8Ghz at all times. Also the GPU is factory OC. That's why it pulls more than 300W from the wall but it gives you some idea.
http://www.hardocp.com/article/2013/07/09/gigabyte_gtx_760_oc_version_video_card_review/9You said you want to use watercooling, that means you need to power a couple of additional fans and a pump. I'd go with a 650W power supply for example "enermax revolution87+ 650w" Your needs will be perfectly covered. Adding an additional GPU requires roughly a 850W supply.
