There's a couple of concerns:
1) power draw - You don't mention your CPU or other components... If you have a 65W TDP CPU, your system might draw 115W or so running Prime 95. If it's a 95W CPU, raise this to 145W or so. That theoretically leaves 155-185W for a video card IF the supply can support nearly 300W from the 12V rail.
Video card TDP:
HD 7850 130
GTX 650 Ti 110
GTX 660 140
GTX 660 Ti 150
HD 7870 175
GTX 670 170
I'd be hesitant to put more than a HD7850 / GTX 660 in there.
2) power connectors: Do you have the 2 PEG connectors for the Video card or do you need to get some molex-> PEG adapters?
3) Single rail or multi-rail PSU design? If multi-rail, then you'd want to use connectors from more than one rail.
4) card length and fit.
5) Thermal and noise: If you opt to run the PSU near 300W, I'd expect the system to run noisy and hot. The PSU is only bronze rated, so it'll add another 60W of heat.
1080p Gaming build: i5-4670K, Mugen 4, MSI Z87-G45, MSI GTX 760 2GB Gaming, 8GB 1866 RAM, Samsung Evo 250GB, Crucial MX100 256GB, WD Red 2TB, Samsung DVD burner, Fractal Define R4, Antec True Quiet 140 (2 front + rear) case fans, Seasonic X-560. 35-40W idle, 45-55W video streaming, 170-200W WoW, 200-230W Rift, 318W stress test (Prime95 + Furmark)
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