How much of an impact does the water have on a beer? How much of an impact does the water (minerals/quality/distilled) have on a beer?
Most of beer is water, so it is vitally important.

Here in Atlanta, we have some of the better tap water compared to many cities, but Monday Night Brewing in particular takes all minerals and chemicals out of the water and adds back in the appropriate properties to match the style of beer that they are trying to brew.

They emulate the water from the origin of the beer style. Just consider that anything but the most pure form of water will impart some taste in your beer good or bad and is worth being aware of.