To get the best results out of Dragon Naturally Speaking you need to make sure that the speech that it’s processing is as clear as possible: which means that you have to use the highest-quality microphone and soundcard that you can get.
Inbuilt soundcards are good, but they're usually designed to maximise the quality of the sound which comes out of the computer, rather than the sound going in; and they’re not often properly shielded from the electrical noise and static that your computer’s other components cause. Consequently, you’ll usually get better results if you use a soundcard that is separate from the main body of your computer.
There are several you can choose from. I use an Andrea USB sound pod, which plugs into any spare USB port and has a jack for my microphone to plug into directly. It is relatively cheap (it costs around $40 in the USA, or £40 in the UK), works brilliantly, and improved my accuracy with Dragon all on its own. Money very well spent.
There are several suppliers, such as KnowBrainer or eMicrophones in America, and the excellent IT Speaking in the UK. KnowBrainer has a good discussion forum too, where you can get lots of help with Dragon, and advice on the best peripheral hardware for your needs.