I think you'll find you'll be wondering what else you can put in your machine.
I'm using a similar Antec High Current Pro HCP-1200, mine seems to look a little different for some reason, to power an overclocked i7-3930K and a pair of GTX580s. I've seen it draw up to 750W, but will regularly draw 660W, while gaming from the wall socket.
I also have a Xeon system with twin E5420 CPUs, this system has a lot of hard drives and a GTX550Ti. This system runs off an ANtec 650W supply. This system is used mainly as a game/file server but when I ran the Intel Burn-in test, it ran to about 300W.
All in, I think your system will draw in the region of 700W-800W. That's 400W for the GTX690, upto 280W for the CPUs (140W per cpu) and 30W-80W for the rest.
EDIT: Just noticed in the cpu specs that's it's Bulldozer and was released June 2012. I have to question why you're being handed a system where the processors are less than a year old and cost £1000+ each, my instinct is that it's broken.
It is about security really. I am doing high end cuda and cpu intensive stuff. The pc will be on for days, probably weeks.
It is going to be in a data center just to have guaranteed power (so using their UPS system as well).
It isn't about really wanting to add on, as it is wanting to ensure the powersupply isn't junk and will crap out after 1 month of full load.