Belkin make a decent iPod 'Y' cable (stereo 3.5mm plug to 2 RCA's), about $30. I have one and it does a great job.
For better sound, Apple make a well-regarded ipod dock for about $100, (uses the line-out of the ipod rather than the headphone jack, bypassing the ipod's volume control) and connect that to any line-level input on your amp.
For better sound, Apple make a well-regarded ipod dock for about $100, (uses the line-out of the ipod rather than the headphone jack, bypassing the ipod's volume control) and connect that to any line-level input on your amp.