Sometime in the future I might add in two more drives (ROG Front Base and another SSD)
Also, i'm going to add in two more 140mm fans for the case.
would the PSU take care of it? or should I get a higher watt one like 1250W?
Yes that would be ideal, but if you plan going 780 SLI you will probably need to upgrade
SLI is having two or more GPUs right?
I won't be needing any more than one GPU so I think I'm good.
so 750W would be enough?
Nvidia recommends a system powersupply of 600watts (for the entire system) with a minimum of 42Amp. on the +12Volt rail, with one 6-pin and 1 8-pin PCI-Ex power connectors.
The Seasonic has the two required power connectors and with 62amps on the 12V it more then meets the minimum requirements.
So, you are good to go with that build, and as per power you will have a good bit in reserve for additional hard drives, fans, etc. If you stay with one card, maybe even a little overclocking.
Okay I am here, if you are not going to have more than 1 GPU I suggest you to stick with 600W. You can look into the Seasonic 620W PSU, it is very good and has good efficiency.. Usually the 780GTX doesnt take more than 450W if you use it to highest potential. To tell you in detail, put your build here and check you will know the optimum power required.