another thing to note, if you use a high quality car amplifier, it will have isolated inputs already, meaning it has a line isolation unit built into it.(well it acts like one anyway) It does this with the type of "floating power supply" that these amps use to get high voltage and current. My Alpine amp runs @ 35V+/- at 3.5A per rail. But because of the switchmode design, the earth point floats.
This basically means, if your amp is a goody, then you won't have any problems running the sound card directly into the amp. A quick test you can do to see if your amp is isolated, touch the rca input terminal (outside ring) with your finger, if it hums/buzzes, then you know that the RCA earth is floating.
The rule of thumb is to keep one end of your rca cable earthed only. So if it is earthed at the computer, then you want the other end to be floating. A good amp does this for you, but you can do this by using a Line Isolation Unit, or disconnecting the sheild from the rca cable on the amp end.
Just something to add....