Ask yourself.. What do I want to do with the final images? and how much storage space do I have? The latter is probably no longer a factor for PC based storage. If it is, then choices are available, including upgrading to a larger hard drive, adding an external USB drive or using this opportunity to purchase a Multimedia PC with more Memory and larger hard drive(s). 

If the target is a personal video player, then with 30G storage space, this could store over 3,500 pictures at maximum resolution or over 60,000! at the lower settings. Therefore one could argue just scan at maximum resolution to get the best quality. Whilst it is strictly true, the reality is computer monitors, photo printers and our eyes can only resolve to a finite level of detail. Hence back to the original question.. What do you want to do with the final images?

Do you want to e-mail or post them on a Web site, display them on your PC or TV, print small photographs, or enlarge them for a poster-sized print? For viewing on a TV or PC, a lower resolution is perfectly adequate. For example, using a 17" monitor set to display at 1024 x 768 pixels, a 6" x 4" photograph, scanned at 300 dpi will already be larger than the screen. [Note: 6" x 4" photograph at 300 dpi equates to 1800 x 1200 pixels]

However a monitor is at the lower end of the display resolution quality ~ 75 PPI (pixels per inch). [Note: 17" monitor is ~13" x 10"; 1024 pixels / 13" = 80 PPI]. Therefore for display on a monitor, the lowest resolution of 300 dpi  will do. In addition a lower dpi setting will also reduce the file size of the image and reduce time it takes others to download or display your image. [Note: for slides the lowest resolution is 1000dpi - 35mm slide at 1000dpi  is roughly 1380 x 880 pixels]. 

Printers, however, can print at much higher resolution. The unwritten rule is to aim for a minimum 200ppi for a good quality print (and 300 ppi for the best home photo printers). So for a good 4" x 6" print we require a 800 x 1200 pixel image. Therefore scanning at 300dpi (& 1000dpi film/slide) will be good for a 4" x 6" but is possibly marginal for a quality 5" x 7" enlargement. As a rough guideline, please see the resolutions vs. the dpi and corresponding reliable printing sizes for the scanning options available below.

For more flexibility and peace of mind for the future, go for the maximum resolution now. The images can always be downgraded in photo editing software to fit the chosen display whereas the converse is not possible  i.e. a 300 dpi image cannot be upgraded later to a better quality 600 dpi resolution.