you could run the magni alone if you wish. its a great amplifier. however, if you do this you would be using the dac that you have already (your onboard audio on the motherboard or soundcard)
if you ran the modi you would be upgrading the dac as well so that you have both a quality dac and amplifier.
is it worth it? depends on how good your onboard dac is honestly but most audiophiles would say yes it is.
be aware.. an external dac does NOT have virtual surround sound built in. to get virtual you only would have the option of using razers surround software... thats it. if you absolutely need better virtual surround then an alternative would be using a xonar dx soundcard ($80) paired with the magni. its a good pairing because the dx...