Yep, those shutter speeds would definitely lead to blurry photos. The minimum suggested shutter speed to avoid camera shake is 1/focal length, so that would be at least 1/50s if not faster.
So, since your ISO is already at 800, in order to get a good exposure you'll probably need to use a tripod, in which case you can drop your ISO down and use those slow shutter speeds (in excess of 1 second). But that does't really help with taking night portraits. For taking night portraits of your fiance, whalebone gave you some very good advice.
(Oh, and I'm just moving this over to the "Other Technique" area
)