There is another girl with a thrunderbird doing what you're doing. I can't tell you exactly why the are doing it that way other than teh seemingly obvious 1) their car came with 4 speakers adn they didn't want to add more, and 2) it was an easier set up than a multiamp one.
You have 6 speakres. I'm not sure about your audio card, but if you have a 6.1 sound option, i'd say go for it for the hell of it. I'm not sure if it can hurt. Get a 6 channel amp at 6.1 surround if your dash speakers are spread apart, or a 5 channel at 5.1 surround if they are close together, and a seperate sub amp and call it a day after wiring it up. IMO, that seems like a decent approach to it