The SB-400 is not much more powerful than the builtin flashes. It can bounce, a little, but it doesn't really have enough power to do that unless you're using a bounce surface that's quite close.
Choosing an exposure such as 1/200, f/8, ISO 200 is not very likely to be successful. Even given TTL flash, you have so many variables, such as focus distance, ambient light conditions, and the like. Frankly I'm very skeptical: most churches are fairly dark, and you're often not very close, meaning that the flash may not carry that far. Can I tell you want to use? Nope.
You do want to use TTL flash, which means through the lens metering AND the flash should be the primary illumination. TTL-BL would mean that the meter should aim for fill flash only, probably not what you want given that you're asking these questions.
I would estimate that you'll need much higher ISO than 200, if it "not very well lit" proves to be correct. As an example, I turned on all of the lights in my living room, such that most humans would call it fairly well lit. At f/8, ISO 200, I needed 1/2 second exposure. If your wedding is in the same light, your ambient exposure of 1/200th, f/8, ISO 200 would be six full stops underexposed, meaning that the flash has to provide six stops of light. I'm too lazy to look up the guide number etc to do the calculation, but I'm pretty certain that the builtin flash or SB-400 are not up for that - especially if you try to bounce. More likely to get reasonably close to possible, I'd start with ISO 1600, f/4.5: the ISO is three stops more sensitive, and the aperture is a stop and a half wider, total 4.5 stops brighter. Then if your ambient light environment is like my living room, the flash only has to provide 1.5 stops or so, which either of the two flashes probably can do up to a reasonable distance.
_____ Brian... a bicoastal Nikonian and Team Member
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