offcourse ur 5.1 speakers will work on creative 7.1 soundcard. the only difference is that creative soundcard has an extra jack for side speakers which makes it 7.1 instead of 5.1....so all u have to do is to ignore that jack as you dont need that and use the soundcard as a normal 5.1 soundcard i.e. by using the front, rear and sub/center jacks present on the soundcard to connect ur 5.1 speakers...
ENjoy...