ISO is not important with digital photography. With almost any digital camera made over the last five years, the difference in "digital noise" (which is the digital equivalent of film grain) is almost unnoticeable with ISO 400 and below.
Yes, if you closely study side-by-side comparisons, you will notice a little more digital noise with each increase in ISO. However, unless you are making poster-sized prints, you will not be able to perceive a difference without close comparisons. And even at poster-size, an increase of one ISO (for example, ISO 50 to ISO 100) will not be noticeable.
That is why I typically use ISO 200 when I can. You cannot tell the difference between it and ISO 100 whatsoever. Even with a close side-by-side comparison, it's very difficult to tell any change in digital noise. Camera manufacturers have done a great job of minimizing noise with low and medium ISOs.
Once you reach ISO 800 you can start to tell a difference. Still, if you only enlarge the image to 8" x 10" (or 8" x 12"), the difference in digital noise between ISO 800 and the lower ISOs will be very difficult (if not impossible) to notice without close study.
The digital noise at ISO 1600 and above seems quite noticeable. Unless you want a "grainy" image (for example, increase the drama in a black-and-white photograph), these higher ISOs should be avoided if you can help it. However, if the print will not be larger than 5"x 7", you probably can't tell the difference between ISO 100 and ISO 1600 without a close study. Even an 8" x 10" print will show very little difference between ISO 100 and ISO 1600, although the ISO 1600 print may come across to some critics (such as yourself) as "just a little too noisy".
ISO 6400 and above is unusable, unless you are purposely wanting an image with tons of digital noise, or the print will never be larger than 4" x 6".
All of that is to say that with digital photography, aperture and shutter speed are more important than ISO. Worry more about getting the depth-of-field you want and motion you want. Try to keep the ISO at 400 or less (unless you are trying to get digital noise for effect), but know that it may not make a difference up to ISO 1600, depending on how large the print will be.