If I always format my memory card for my Nikon D5000 well that affect the picture quality?

I have been doing this for about a year now and a few of my pictures I found after I import them to Aperture 3 the bottom half of the picture is gone.

I never format my memory cards i just take pics over the old ones i have put in the comp
get a new card they are cheap

I always format my card after I move them to my computer. It's recommended that you format your card every time you delete everything on it. Losing the bottom half of some pictures is from something else. Just import them again.

Formatting the memory card is a preventitive measure to protect it from becoming corrupt.

People who delete images to make room for more end up with corrupted cards.

As a pro, I ALWAYS format my cards after I have 1) copied them to my computer and 2) have made a DVD backup of all the files.

What can cause the bottom half of an image disappear is caused by not using the correct flash sync speed, but NOTHING to do with formatting your card. Look in your cameras user manual to discover what that shutter speed is.

Usually it is either 1/200th second or 1/250th second.

It is possible for a memory card to fail, although thankfully it is not really a common occurrence in the name brand legit cards. Beware cheap cards from the famous auction site, it is loaded with counterfeit cards.

I format after every shoot, after I have the files downloaded and backed up. I always format in the camera. I seldom delete an image from the card using the camera delete button, I do the edit after I transfer them to the computer. I have some cards I have been using for several years, both CF and SD.