If you really want an ultra-high quality cable, check out the Silver Dragon ($65) cable in this thread:
Ipod + aux input
Also mentioned in the thread is a cable that's produced by Cardas. I checked out the Cardas cable in GregM's car. We compared his cable to a generic cable. The Cardas cable really brings out the mids and highs.
There are lots of other quality cables out there that can be had for around $15 to $20 for a three to six-foot cable. Most cables are just generic copper wires that are branded with a well-known name.