Flash is the worst thing you could ever do with a night shot. If your camera can do it, turn off the flash, get a tripod, and increase the length of time the shutter stays open. Cable release wouldn't hurt either, if you're camera will accept one... anything you can do to avoid touching the camera when you initially take the shot, and while the shutter is open.
The longer the shutter is open, however, the more grainy the picture will look. 1 - 5 seconds exposure should be more than enough for night shots with decent ambient lighting (such as the shots you've got), and shouldn't produce noticible graininess. Sometimes if it's just too dark even for the ambient light and you have to keep the shutter open 10+ seconds, it just looks horrible. I'm not sure why digital camera's do that. My 35mm SLR can have the shutter open as long as I want, without the film being exposed getting grainy (unless it's 400+ speed film). I have a few shots I took during a lightning storm, had the shutter open about 1 - 5 minutes at a time. Never did get a good shot though, didn't have a decent clearing to see the lightning over the trees.