@OP, did you mean the Z5500? Google searching a Z5000 model yields no results.
Assuming, you meant the Z5500, you can go either the Analog route or the digital route.
If you're going to use a digital connection (suggested), get the Xonar DS or a Xonar DX. Why? Because the Xonar DS has DTS Connect, and the Xonar DX has Dolby Digital Live, either of which will convert any multi-channel audio from your computer to a digital signal that can be decoded by the speakers - this allows gaming in multi-channel over a single cable. Some motherboards can carry either or both of these features. If this is the case with yours, you can skip the sound card altogether. However, if you're not interested in gaming, then forget about the sound-card and connect the digital cable (optical or co-axial) to the motherboard - it'll surely have one or both, unless your board is really old or really low-end.
If you're going to use the analog connection, then you'll be bypassing the DAC of the speakers itself (not suggested), so quality will be dependent on source, but even in this case, I will not suggest buying anything over a Xonar DG - the cheapest Xonar card (does not have either DTS-C or DDL), as the upgrade in sound quality by purchasing pricier sound cards will be, IMHO, minimal to non-existent.
In any case, enjoy your new speakers!
