Yep, I'd always take as many pictures as I could because, that way, you're playing the percentage game and are likely to get some that work. Having said that, if you're going to be expected to come up trumps with a free album for the bride and groom, I'd make sure they realise that you don't have high hopes of being able to capture the event under those conditions.

Shooting at ISO 400 should be fine, ISO 800 will probably have noticeable grain and anything above that might be problematic, depending on your camera. I'd shoot in aperture priority, so I could set the minimum shutter speed that I thought I could hand-hold (around 1/30th of a second perhaps) and then let the aperture look after itself, but knowing that I'd probably not have a lot of depth-of-field.