200W is pretty weak. It depends on what other components you have in there, and how much everything uses up total. The processor, hard drives, other peripherals and cards in the PCI slots. You could try it and see. If you have problems you can always grab a decent 300watter.
You gotta figure 200 watts at 12 volts is a little over 16 amps.Thats a lot of current. On the other hand you also gotta figure that it's not 100% efficient, and that it won't run too well at 100% capacity, if at all. And a decent 300 watt supply isn't all that expensive. Up to you what you want to do.
Bottom line is it might work, but if ever you should exceed its capacity, or heat should stress it to the point of failing, you could end up losing precious data. Gotta figure ain't no UPS backup to fall back onwhen the power supply dies. OTOH if you're careful and lucky it could work. figure if you have 8 or less components all running at less than 2 amps each you're ok. But like I said before running at 100% capacity, or near it, is pushin your luck.