i would think that your 7.1 sytem would have 4 cables seeing as a 5.1 system has 3 (ctr, fl/fr, rl/rr). you might want to look at your specs to see which cables have which function (check your speaker system manual).
pretty much any soundcard you buy that is newer should have drivers included for windows 7. if not then there should be updates available. if you are unsure just look at the model you are thinking about buying and look (search google) to see if its win7 ready. if it doesnt come with the new drivers you might also be able to download updates.
whatever card you are interested in you should do a little research. honestly its the same thing i'd have to do to answer your question so you might as well have a quick look yourself.