thx certified....sounds like your motherboard already has an onboard sound card.
will you notice an improvement? maybe a very slight one. you will notice the different drivers more. i liked the soundblaster drivers back when i had one.
its only worth buying a soundcard if it has features that your internal one doesnt provide. ie...dolby surround decoding, less noise, etcetera. i've used both internals and externals and as long as its not a junker theres usually no issues.
the soundcard does all the decoding. all the speakers need to do is be a speaker.
as i said...if you require the extra features of a new soundcard or you have a really bad internal soundcard now...then perhaps you want to upgrade. if you dont have these issues then you most likely do not. another reason for upgrading would be if you wanted 5.1 sound (when you buy new speakers) but your current soundcard doesnt offer it.
generally speaking...any of the logitech speakers above $40 should be more than adequate for most people. not the best speakers ever invented but definitely a good value. they provide enough power for most users.