A specific question came this morning: if ISO 800 is used on the Pentax K-x, can I still get good looking prints?
The answer depends on how large the prints will be, and also how critical you are of digital noise (there are some photographers that like digital noise--lots of it, in fact--and others that hate it with a passion). Another factor may be how much you cropped the image (if you cropped at all). And resolution definitely plays a factor--the larger the sensor, the better.
Generally speaking, though, you can get quality 8" x 10" (or 8" x 12") prints using ISO 800 (and that is with pretty much any modern digital camera, not just the Pentax K-x). At that size, you likely won't be able to tell the difference between ISO 100 and ISO 800 without a close side-by-side look (and, really, outside of "pixel peepers", who does close side-by-side studies?). If you didn't crop the image and you used a small amount of noise-removal in post-processing, you could get quality 11" x 14" prints. Depending on the camera, you might be able to get even larger prints and still not notice the digital noise. With the Pentax K-x, 11" x 14" is probably the largest print you can get before you start to notice the digital noise without a close side-by-side study. Again, if digital noise doesn't bother you, or perhaps the print will be viewed from at least a few feet away, you could go larger.
If you are trying to avoid digital noise at all costs, my recommendation is to keep the ISO at 400 or less. Still, at ISO 800 you should be fine as long as you're not making poster-sized enlargements.