What he said ^.
Think of your car after it is driven, there is no condensation on the hood unless you let it sit for an extended period of time.
This is because the hood temperature drops below the dewpoint, meaning the water can no longer evaporate faster than it condenses on the hood.
If you are running the outside air through the entire enclosure I would worry about condensation in cooler areas of the case.
If you could run the outside air over only the video cards you should be safe since they likely put out enough heat to keep a small enclosure above the dew point.