If you are going to be playing any games on tis PC then I would def go for an XFI. You can't beat them for games. They use EAX5 when all the other cards can only use version 2.
If you are only going to listen to music then I would suggest getting a different card altogether. Therea lots of cards that are better at musix than the XFI's. For example, the Auzentech X-Meridian 7.1 Soundcard is alot better than the XFI for music, especially as you can upgrade the OP-Amps on them to sound even better.
The only time I wold go for onboard sound is when it is connected to a decent amp and you output the signal through the digital output. Again I wouldn't do this for games though.