If you are using subsurfs set the 3d window subsurf value to 0 (the left of the two values, right one is the rendering subsurf value). Make sure your not in shaded or textured mode. Solid mode is pretty fast but wireframe is the fastest. In some situations I have even resorted to just using bounding boxes for parts of the scene I wasn’t interested in. Put some of your animated objects on another hidden layer while your working on other parts of the animation. You can use scenes in some situations to speed things up as well (composite them in the sequence editor later).
You can also copy one of your complex objects and use the decimator to reduce the poly count. Link this mesh temporarily to your complex object. You will have to link the other (complex) mesh again before rendering however.
I too have an older machine PIII 450 with a geforce2