Try some math to see how many pixels you may want at those sizes.
A "guess" is that you want 300 DPI. So for 4x5 prints with NO BORDER, that's 20 square inches and 90,000 pixels in each square inch. That's all of 1.8 million pixels so at 3 MP, you should see good prints.
I have recently purchased a Sony Cyber-Shot Dsp-10 5 megapixel camera. I know I have the option to shoot at lower megapixels so I can take more pictures on my 128 memory stick. If I take pictures at 3.2 how will it effect the quality? I usually order 4x6 prints from Shutterfly, will the quality still be there as long as i do not blow it up? Thanks for any insight.