i7 2600k - 95W
ATI 6850 - 125W
ATI 5850 - 150W
Your current power usage are already conservatively at 370W.
Putting a 200W GPU in there, would be putting yourself that bit closer to the max of that power supply.
Unless it's a very high quality one, it most likely will not cope with the power requirements.
You need more wriggle room, as power supply's are only about 85-95% efficient and over using one voltage line, will go over what it is rated for.
In your position, if your consider putting very powerful multiple GPU's, I'd only do so with more info about your power supply.
Wrong.
I don't know why PSUs are the most consistently misunderstood pieces of hardware, but I see this so often, it hurts.
Quality PSUs are rated at XXXW Continuous, meaning they can deliver XXXW 24/7. Non-quality PSUs are junk and shouldn't be used at all.
A PSUs efficiency has nothing to do with what it is rated to deliver, it has to do with conversion efficiency (your PC uses DC, and your house delivers AC, there is an AC/DC conversion, and there is where efficiency comes into play).
2 7950s on a 600W PSU should be fine, a 5850 and 7950 even more fine (even though sometimes they do not play nicely together), but do maintain care that you are not aggressively overclocking, as your system (CPU, mobo, HD, fans, little LED lights, beverage warmers, etc) all take up power as well, and you do want to leave the "wriggle room" above to make suer you don't blow nothin.
