So you can't take your old CDs that you don't listen to any more and burn over them. The speeds of a CD are 2x, 4x, 8x, and 16x. This is how fast the CD is allowed to burn at. Tucker says that usually the rule of thumb is that the slower you burn a CD, the higher quality it will be. If you burn faster than 4x speed, there is a higher chance that the CD will not play in a CD player in your stereo or car.
Many CDs are able to burn at multiple speeds, but be sure to check the label to see.
If your burner burns at 2x speed, then don't buy CDs that can only burn faster than that. There are typically two kinds of burners, computer burners, which are connected to a computer or in a computer, and stereo burners, which can be in newer stereos or in a stereo tower. Both types of burners have pros and cons of course. Computer burners are great for burning, but typically cause the most stress. Computers are complicated machines and many things can go wrong.