After downloading the official drivers, it seemed to me that the Terratec 512i did not function properly (and I'm sure I used the right installation procedure). The sound was fading away, quality in general was minimal.
If you want to keep your Terratec 512i running with windows XP, you will need to use the FM801 drivers, win2k-xp-eng.exe, somewhere to be found on the web, for instance on http://www.soundcard-drivers.com/companies/409.htm
Took me a long time to find out ! The card has a right sound probably only if you use the Forte Media drivers.