They both should work fine together.
X-Fi has standard stereo line output so connection is not issue.
It is probably better to use analog out from the card for it should have better sound quality then DAC in your speakers.
But you might use digital connection if you wish (you will need to
1) set X-Fi "flexijack"to digital out and
2) connect it with a 1/8-inch-jack-to-RCA-cable to the digital input on the bostons.