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A tripod makes it way easier to use a slower shutter speed to let in more light, and get a movement effect without blurring your picture. you also may be able to avoid changing the ISO like that.
another good way to keep from shaking is using the timer on your camera (if it has one)

DPreview did a comparison between these:
# Nikon D300:

# Canon EOS 40D:

# Sony DSLR-A700:

# Olympus E-3:

With this result:

From ISO 100 to 400 there is virtually no difference from a noise point of view between any of these cameras, none exhibit noticeable any noise. Indeed visible noise only begins to creep into the D300 shots at around ISO 1600, at which point it has a very 'natural' monochromatic grain appearance (not want to be too cliched but 'film like'). At ISO 3200 the effects of the D300's noise reduction are more obvious if you look closely at the detail crop. ISO 6400 really is only for the brave or those emergency situations. It has to be said that out of these four cameras the D300 has the nicest appearance to its higher sensitivity images, Nikon's decision to preserve as much luminance detail as possible by using mostly chroma noise reduction really pays off.

Link to test:
http://www.dpreview.com/reviews/NikonD300/page18.asp