I think you'll find that a large percentage of users here are running without head units.
As far as amp recommendations, it's a matter of personal preference, what kind of power output you're looking for, and the amount of money you have to spend. Go with any good quality amp and you shouldn't have any problems.
For sound cards, some people are happy with the on-board sound (if your motherboard is so equipped), while most seem to prefer an add-on card or USB solution. The advantage of going with a USB solution (like the Audigy 2 NX) is that you can put it close to your amp, and therefore are less likely to have interference problems from long cable runs.
Look at the MP3Car store for the HQCT which, by all reports, seems to be a good solution for terrestrial radio. I plan on getting one for my setup.
By the way, a quick search would have turned up answers to all of your questions.