I have used extension cables and since I have a 4 port KVM switch box I have experienced the same ghosting problem as you, if I use inferior cables. I am in UK so I haven't used any of the cables woodie1 mentioned, but I can say that high quality cables are definately available and usually cost twice as much. Best to go for the best you can afford.
What resolution are you running? - the higher the resolution the greater the effect on quality if you add an extension cable.
Out of interest, why do you need an extension cable (I guess so that your CPU will reach your monitor ;-) but why)?