Brianna-
Actually this is a good place to start, or better still probably the Photography forum at <
http://www.adobeforums.com/cgi-bin/webx?14@@.3bb41392>.
The ACR Forum logically is ACR-centric and like Ramón suggests has good info on using Adobe ACR to convert your RAW images. However there are many other solutions for converting Nikon NEF photos. Many pros find that Nikon CaptureNX2, Capture One or Apple’s Aperture provide visually superior NEF conversion to Adobe’s ACR. Individual cameras and individual preferences do vary.
Unfortunately with a G4 iBook the limited hardware power is a significant limiting issue. And OS 10.3.x excludes almost everything that will drive RAW conversions from a modern camera like the D40. Probably you are forced into an (IMO inferior) ACR2.4/DNG RAW conversion like Ramón suggests, or for likely far better RAW conversion quality you can investigate using Nikon ViewNX, which I believe came free with your D40 or can be downloaded from Nikon, and alleges to run OK under OS 10.3.9.
IMO Nikon’s software is a bit clunky, but only Nikon has full unimpeded access to the RAW data of Nikon cameras, so you will probably like the resultant RAW conversions of ViewNX if it runs adequately on your setup. Do a trial.
These suggestions are in addition to not in lieu of Ramón’s ACR suggestions, so try them too.
Good luck!