Hi,
If the 5.1 amp you buy has an optical input or analog input (two phono cables) then yes you will get 5.1 sound from your Xbox, if the amp is capable of upscaling the sound to 5.1, if you know what i mean. The two phono cables only carry stereo sound, so the amp will have to do some magic to convert it into 5.1. So the sound will not be brilliant 5.1 i don't think. I have the Creative Inspire GD580, which is the 5 speakers + sub and a 'box of tricks.' I have an Xbox, pc, and freeview box hooked up to this and it gives me decent 5.1
Hope this helps