When buying a digital camera don't become a sucker to megapixel marketing. Almost all digital cameras today have enough megapixels to satisfy the average consumer, however more megapixels don't always mean a better quality picture.
For photos you plan to only use on the Internet or send by e-mail, you can shoot at a lower resolution. If you know you want to print the photo, you'll need to shoot at a higher resolution.
Determining how much camera resolution you'll ultimately need for a print depends on the size of the print you want to make. The table listed below should help you decide on the proper resolution.
Source: http://forums.adoramapix.com/entries/228318-Resolution-and-image-quality
Camera
megapixels Approximate standard image resolution in pixels
2 megapixels 1600 x 1200
3 megapixels 2048 x 1536
4 megapixels 2274 x 1704
5 megapixels 2560 x 1920
6 megapixels 2816 x 2112
7 megapixels 3072 x 2304
8 megapixels 3456 x 2304
10 megapixels 3648 x 2736
12 megapixels 4288 x 2848
Print size Optimal Resolution Minimum Resolution
For good print quality less optimal image quality
300px per inch 100px per inch
3.5x5 1500 x 1050 500 x 350
4x5 1500 x 1200 500 x 400
4x6 1800 x 1200 600 x 400
4x12 3600 x 1200 1200 x 400
5x5 1500 x 1500 500 x 500
5x7 2100 x 1500 700 x 500
6x9 2700 x 1800 900 x 600
8x8 2400 x 2400 800 x 800
8x10 3000 x 2400 1000 x 800
8.5x11 3300 x 2550 1100 x 850
8x12 3600 x 2400 1200 x 800
9x12 3600 x 2700 1200 x 900
10x10 3000 x 3000 1000 x 1000
11x14 4200 x 3300 1400 x 1100
12x12 3600 x 3600 1200 x 1200
12x18 5400 x 3600 1800 x 1200
16x20 6000 x 4800 2000 x1600
20x30 9000 x 6000 3000 x 2000
24x36 10800 x 7200 3600 x 2400