The advantage of digital is of course you can take as many shots as you want for free.
Last year I didn't have an SLR, just a pocket digital which did shoot video but only at 640 x 480 resolution. I did capture and post a few bolts caught from afternoon storms. We didn't really get any night time ones last summer (or this summer).
I now have a good Canon 1000D with full control. I can shoot out into the backyard with the ambient light at night for about 20 seconds at 100 ASA with the lens wide open. Unfortunately there has been little to try it out on. The last crawler I caught used that setting, the first one I ever managed was really quick and I had to set to 800 ASA for a proper exposure.
What you need to determine for your location is how long you can expose for before the streetlights and reflected lights off the cloud start to wash out the background. That will vary from place to place. Sydney obviously being a lot worse than Ballina. If I was still living on the farm at Coraki I could probably do a 5 minute exposure.
Then for most lightning - certainly good ground strikes I believe 100 or 200 ASA is ideal. Once you catch the first one you can preview and see if you need more or less light. It will depend on how far the lightning is from you as well as how bright. Obviously a multi strike in one spot lets a lot more light into the camera.
Check out some of the photos posted here, they have the EXIF information included so you can see what other people used.
