I paid just under £200 for an elite pro to go with my Z5500s and was VERY underwhelmed.
Sold after less than a month for a loss.
Now looking at the ASUS Xonar HDAV1.3.
Personally, I would say up your budget and go for one of the Xonars.
The X-Fi chip really isnt all that, sure it is better than onboard audio but your Z5500s deserve something better.
Why pay £200 for a pair of speakers then run them off a £40 soundcard? Madness.