Guys ...help me in gettin a Sound card

I've gota 5.1 multimedia speaker logitech x540. and i'm now in a confusion where to go on for a 7.1 sound card or a 5.1 card...
my Mb don hav an inbuilt 5.1 sound card..but plannin 2 buy a new 1..

if i get a new 5.1 card will it go useless vn i get a new MB ??

or shall i get a new 7.1 card n use this 5.1 speakers n l8r upgrade them ??