1) a good sound card: Auzentech makes some great ones. One with a Burr Brown OpAmp is what you may want
2) amount of amps: that all depends on how your speakers are setup...but with 3 sets, i assume front middle of car and rear...so you need 2 for those, and one for the sub making that 3....or you can dedicate an amp per set of speakers and maybe bridge them for extra juice if they are 4 channel.
3) a way to control each speakers eq and such as best i can with the actual computer: you definately want an Alpine PXA-H701 and a controller like the RUX-C701. 31 band equalizer per channel and it can do 5.1 if you wish....and it has digital optical inputs for the most amazing sound...I have one, but i havent fully installed it yet...but i have seen one in action, and its amazing...