I wouldn't expect to notice much difference in performance unless your system is struggling already unless you're using a USB headset. The difference might be noticable going from that to a sound card, but I still believe it would likely be minor.
Channels only matter as far as what types of setup they will support. You can go down with your number of speakers, but you can't go up. So if your card will do 7.1, you can set it up as 5.1, but you can't set up a 5.1 card to do 7.1.
5.1 means you've got 5 speakers, plus 1 subwoofer. The channels are center, front left, front right, rear left, rear right, sub. A 7.1 setup has all of those, plus an additional set of speakers straight to the left and straight to the right of the center.
edit: In my opinion, if your PC already has onboard 5.1 or 7.1, it's not worth the cost of an additional card. YMMV of course.
Wiley.