I think a sound card is quite important above a certain threshold, you need something that goes with your other equipment. I'm currently using the DAC built into my AVR, and PCM from my graphics card, which is absolutely fine. If I was using analogue linkup however, I 100% would have kept my sound card, as it DOES make a difference, however if you have weak speakers you will barely hear it as your output isn't strong enough.
If you're using £1000/$2000+ dollars of audio equipment (hell my amp alone cost that much if I'd got it when it came out), then it makes a hell of a difference. Onboard is definately better than it used to be, and nowadays I'd recommend going for a hi-fi amp and speakers before a sound card, if yours is OK, as the speakers tend to be the biggest 'weak' link. No Logitechs, no matter the cost, do not sound good, the manufacturer of the driver actually rates them about 1/5 the wattage Logitech does...see the way this goes? (In general most PC speakers produce shrill treble and boomy inflated bass, with p**s weak middle)
Obviously every link in your system has to be comparable or like a PC system you end up with a bottleneck in your sound quality, however spending some cash and a bit of time to research decent second hand equipment, and re-ripping your CDs to Q0 VBR MP3s with a LAME encoder is certainly worth your time, if you play a lot of music, or care about sound quality. If you don't fine.
For the record I have upgraded my car audio before, a fiesta sounds HORRENDOUS as stock! I haven't changed more than the headunit in my current car for financial reasons though.