Onboard sound is more susceptible to noise than even internal soundcards (although, if you stick an internal soundcard in the slot under a video card, expect static).
I've seen the M-Audio Audiophile reccomended a lot recently on some of the other forums I frequent, mainly for it's external firewire interface while still keeping ASIO (extremely low latency audio in/out) but not all motherboards have firewire so that could be a problem.
Unless you require crystal clear audio with no pops/interference (i.e. if you dispise radio), it's probably not worth buying an external soundcard.
If you hate interference, but aren't really obsessed about the audio being perfect, probably go with a USB/Firewire soundcard in the boot of the car so there's minimal cable length between the soundcard and the amp.
If you can't stand listining to anything less than a $3,000 stereo, and/or own $200+ headphones, you'll probably want to look at a digital processor like the Alpine PXA-H700/701. It'll take RCA or digital optical in, and output at 4V on as many channels as you want, with as many options as you want (time alignment, crossover adjustment, EQs for every channel, etc, etc). Other options would be an amp with digital input. Digital signals are not susceptible to interference of any kind, as they're data, not voltage. They're also higher bitrate than ALMOST any source you can come up with (650k?) except maybe DVD-A.
//Edit: Red's signature is a good example. That's a few thousand on frontstage speakers alone, another $600+ on the Alpine proc, god knows how much on the amplifiers. If you intend to spend that much on audio, you need a processor or digital amplifiers.