Higher speed film will let you use shorter exposure times, but if you're using a tripod the exposure time won't really matter. 800 film, especially when exposed for a long time, I would expect to produce considerable grain. I have used 100 speed film (Fuji Reala) to do night shots before and they came out great. Some of the shots even had nice car headlight trails in them.
Night shots, even at 800 speed, will have long enough exposure times so that you cannot get crisp shots without a tripod. The general rule of thumb that I have heard is that the slowest hand-holdable shutter speed that you can generally use without creating considerable blurring is 1 divided by the focal length of your lens. For example, with a 50mm lens, you shouldn't really use anything slower than 1/50th of a second without some kind of additional support. And even with a tripod, to keep your shots steady, either a cable release or a self-timer in the camera will be useful. Even when on a tripod, the simple vibration caused by you pushing down on the shutter button will cause enough camera shake to create some noticeable motion blurring. Even small movements of the camera during exposure time can cause more blurring than you might think.
I hope some of these tips help.