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The 8mp camera is not the problem. It is probably an issue with previews. It sounds like you have Lightroom set to create minimal previews on import, and that's what you are seeing in the thumbnails. (I would have thought minimal would still be acceptable for the small thumbnails with, say, 20 on the screen, but if you increase the size they will certainly look bad.)
I don't remember exactly where the setting is, but follow the Kelby book to generate 1:1 previews on import. Note that Lightroom will do that in 2 stages. The first just imports the images (probably with minimal previews), then the second step generates the 1:1 previews. It may take a while on a slow computer. Once the second step is done, then go look at the images. You may not want to sit there and watch the computer work the whole time, but if you did you would see in the upper left of the screen the two processes so you would know when it was done. See if that is better and let us know.