The D70 does have a bulb setting, the longest shutter speed in manual mode is 30 seconds but if you have a wireless remote (less than 5 quid from Argos) you can click the shutter open in bulb mode and then click it closed again after as long a period as you want. The D70 doesn't have a wired remote or threads for a traditional shutter release so you need the remote.

At ISO 200 the long exposures are fine, a lot of people use ISO 1600 at night but I have no iea why you would do that if you have bulb mode, maybe thats where the noise issue comes from. I have very little trouble with noise at 200 and I have seen very long exposures with the D70 that look fantastic, here's one at 21 minutes! http://forums.dpreview.com/forums/read.asp?forum=1034&message=14963640