If ur not getting creative speakers, and you dont want to use the analogue outputs on the x-fi\audigy, get the bluegears xmystique which does real time dolby digital live encoding. You can then hook up the z5500 via optical output. That would be a worthwhile uprade.
Other than that, if you are set on the x-fi, just keep your z5300's and hook them up to the x-fi's analogue outputs. You will still get 5.1, but the x-fi will be doing the decoding (not all sources are encoded anyway, only some movies and some audio DVD's). Remember, the only benefit of the z5500 is its decoding capabilites, and to use them, you need to feed it with an "encoded souce", which the x-fi cannot provide.
So basically, if you get the x-fi, stick with the speakers you have, if you want the z5500's, get a soundcard which can do dolby digital live encoding.
Heres an info page on the hda x-mystique:
http://www.bluegears.com/xmystique.html